One of the most common things we do to our cars is change their oil. But do we really understand what and why we are doing or do we just go through the motions like we always have? For most of us, we usually change our oil every 3000 miles and either use the most economical oil or the best protecting oil. Let me propose that we can do both. First off, we should understand some key concepts.


The most basic concept in motor oil is viscosity but many people don't really have a good understanding of this concept. From an engineering point of view, viscosity is a measure of a fluid's internal resistance to flow, more commonly understood as a fluid's thickness. Viscosity can be measured with a variety of units and a commonly used unit in modern oil spec sheets is the centistoke (cSt).

Motor oil is classified according to its SAE (Society of Automotive Engineers) viscosity rating and most people would be familiar with viscosities such as SAE 10W-30, SAE 5W-30, SAE 30, etc. Standard SAE J300 classifies these viscosities and, without getting into too much explanation, a multigrade oil (like SAE 10W-30) has viscosity characteristics of SAE 10W oil (W oils are tested at low temperatures) as well as that of SAE 30 oil. An oil with a single viscosity number (like SAE 30 or SAE 10W) only meet the one part of the standard. The viscosity specifications of SAE J300 are summarized in SAE Viscosity Grades For Engine Oils.

SAE J300 evolved over the years (first introduced in 1926) and the low temperature portion of the standard was added in 1950. Prior to 1950, motorists selected their oil based on ambient temperature. Multigrade oils were developed to overcome the need for seasonal viscosity changes and motorists could instead plan their vehicle maintenance by mileage only.

The following photo gives a good illustration of the differences between oils of various viscosities. All have been cooled to a low temperature and and they were all tipped at the same time. The 15W-40 has barely started to flow while the 0W-30 is nearly completely finished.

Engine Oil Viscosity Comparison

Viscosity Index

The viscosity of engine oils (and all fluids) changes with temperature. The measure of a lubricant's change in viscosity with respect to temperature is its viscosity index (VI), which is an arbitrary scale. It was originally based on the viscosity changes of the best and worst mineral oils of the time, with VI=0 having the greatest change in viscosity and VI=100 as having the smallest change in viscosity for the same change in temperature. The viscosity index today is based on viscosity measurements at 40°C and 100°C.

Viscosity Index Improvers (VIIs) are added to oils so that the oil performs like a low viscosity oil when cold and a higher viscosity oil when hot. A VII is a polymer additive that is coiled up when cold but uncoils with increasing temperature. For this reason, low weight (like 5W and 10W) base stocks are used in multigrade conventional oils. Because of the molecular nature of VIIs, high shear (as found in meshing gears) and high temperatures (as found at the ring grooves) can break down the VII molecules so that the oil loses its viscosity.

High-Temperature, High-Shear (HTHS)

Within highly loaded bearings, engine oil has a temporary loss in viscosity and ASTM D-4683 is test method which is thought to be representative of the condition encountered in the bearings of automotive engines in severe service. Standard SAE J300 places minimum requirements for HTHS for each viscosity grade of engine oil. Generally, the higher the HTHS rating of an engine oil, the better the protection of journal bearings. HTHS can be thought of as a measure of the VII capability of an engine oil. The minimum HTHS for a all 20-weight oils is 2.6 cP while the minimum HTHS for a all 30-weight oils is 2.9 cP. Starting with the the J300 NOV2007 version, for winter 40-weight (0W-40, 5W-40, 10W-40 grades) oils, the minimum HTHS is has been raised from 2.9 to 3.5 cP. For all heavier grades of oil, including HD 40-weight (15W-40, 20W-40, 25W-40, 40 grades), all 50-weight (0W-50, 5W-50, 10W-50, 15W-50, 20W-50, 25W-50), and all 60-weight (0W-60, 5W-60, 10W-60, 15W-60, 20W-60, 25W-60) oils, the minimum HTHS is 3.7 cP.

Pour Point

When selecting an engine oil for winter use, the owner's manuals of early cars would specify something like: During winter, all oils used should have a pour point or cold test below the lowest anticipated temperature that will be encountered during its use. The pour point of an oil is the lowest temperature at which it will still flow. This was an issue with early engine oils because the oil would contain paraffin wax which would start to crystallize at low temperatures. Pour Point Depressants are additives that inhibit the formation of wax crystals. See Pour Point Measurement.

Service Classifications

Another important concept is the API (American Petroleum Institute) Service Classification which is covered in detail in Automotive Lubricants Reference Book (by Roger F. Haycock, Arthur J. Caines, John E. Hillier). Prior to 1885, animal and vegetable oils were used for lubrication. Mineral oils gradually replaced those lubricants so that by 1900, mineral oils were often used in automotive engines. Motor oils contain a wide variety of additives (anti-wear, detergent, dispersant, corrosion inhibitors, etc.) to improve their performance. The service classifications essentially define the types and amounts of additives in the oil to meet performance criteria.

Originally, engine oils did not have any additives and, from 1900 to 1930, oils were selected based on the manufacturer's reputation and viscosity. Because these oils did not contain any additives to improve their performance, they are commonly referred to as non-detergent oils. The evolution of gasolines as well as the evolution of automotive engines required the evolution of engine oils with their corresponding additives. In 1952, API introduced the engine service classification system which consisted of 6 categories: ML (Motor Light), MM (Motor Medium), MS (Motor Severe), DG (Diesel General), DM (Diesel Medium), and DS (Diesel Severe). These categories were revised in 1955 and 1960.

In 1971, the S-categories were introduced which superseded the M-categories. Non-detergent oils (ML/SA) were used until 1930 and, although the API Service Classification System did not begin until 1952, typical engine oils used from 1931 to 1963 were equivalent to a MM/SB. MS/SC was specified for 1964-1967 vehicles (MS/SD was specified for 1968 to 1971 vehicles) and was introduced to meet the performance requirements of the new multi-cylinder engine sequence tests, which were representative of consumer driving conditions of that era. Therefore, ML became SA, MM became SB, and MS became SC/SD. Similarly, the D-series diesel oil classifications became C-series. However, these classifications often branched out within a service category (e.g., CI-4) and, in some cases, a higher service level did not necessarily supercede a lower one.

Every 5 years or so, the performance required by engine oils has been revised with an increase the second letter of the category so that the latest oils commonly available at this time are SM and SN. Similarly, diesel oils have evolved so that the latest categories available now are CI-4, CI-4+, CJ-4, and CK-4. Other than non-detergent engine oils, oils meeting only obsolete Service Classifications are unavailable.

API Starburst Oils

Many automobiles require the use of API Starburst oils. Starburst oils indicate that the oil meets the most up-to-date requirements for passenger vehicles as outlined in the latest ILSAC specifications. The API Starburst mark indicates that the oil meets the warranty requirements of the OEMs and they include friction modifiers for improved fuel economy. Heavy Duty Engine Oils (HDEOs) do not have friction modifiers and fuel economy with these oils will be slightly lower than with Starburst oils. Although reduced friction is an desirable performance characteristic of an engine oil, there are trade-offs with fuel economy and engine protection. The fuel economy improvement of Starburst oils are significant to the OEM trying to satisfy CAFE requirements but it may be difficult for the vehicle owner to measure the difference in fuel economy between a HDEO and a Starburst oil.

Base Stocks

Engine oils have historically been mineral oils that have been refined from crude oils and Pennsylvania crude oils were often favoured for engine oil. Over the years, engine oil technology has improved dramatically and the base stocks used are now categorized into Groups. Groups I, II, and III are mineral oils that are progressively refined to a higher degree to eliminate impurities and varabilities in their molecular composition. Group I is solvent-refined oil commonly used until the 1980s but would now be considered a poor engine oil by itself. Group II oils are commonly used as conventional oils in modern vehicles today. Group III oils are considered to be synthetic oils now because of the severe amount of refining required to create them. Synthetic oils commonly found on the shelf at major retailers like Wal-Mart and Canadian Tire are generally Group III oils.

True synthetic oils are considered to be Group IV and V oils. Group IV oils are PAO (Polyalphaolefins) and Group V is everything else that is not mineral-based or PAO (such as POE - Polyolesters). Originally, Group IV oils were known for leakage due to their effect on seals but they no longer have this problem due to improved additives.

For multigrade oils, a conventional mineral oil (often called "dino" oil) starts off as a low viscosity oil (like a 5W or 10W) and polymer VII additives are added so that it performs like a 30 or 40 weight oil (ie, 5W-30 or 10W-40). In contrast, a synthetic multigrade oil is formulated as heavier weight oil (like SAE 30) and its molecular structure is manufactured so that it performs like a lower viscosity oil (like 5W) when cold. A synthetic oil accomplishes this with minimal or no use of VIIs.

Another advantage with synthetic oils is their high temperature stability. Although the oil temperature in the sump will generally not exceed 212°F (100°C), the temperature in the piston ring grooves can be much higher. Cars like the Saturn SL series (1.9L engine), for example, are prone to to carbon build-up in the rings. A synthetic HDEO oil will resist carbonizing in the ring grooves better than a conventional passenger car mineral oil and will prevent the engine from prematurely burning oil.

Historical Requirements

The engine oil requirements for cars from the 1930s to the 1960s have remained very similar (see Chrysler Engine Oil Recommendations for more information). Basically, most cars required a 30 weight oil for normal operation at ambient temperatures above 32°F. In viscosity terms, this means that an engine at operating temperature will require an oil with a viscosity in the range of 9.3 cSt to 12.5 cSt (@ 100°C). Engine oil relies on the oil pan and oil filter for much of its cooling and its operating temperature is affected by ambient temperature as well as the temperature of the engine's cooling water.

Because the viscosity index of older straight oils was relatively low, it was necessary to use lower viscosities with decreasing temperature. For example, a 38 Dodge D-8 (218 I-6) normally required SAE 30 oil in temperatures above 32°F. SAE 40 was recommended for average daily ambient temperatures of 90°F. Oil specifications for 1938-era oils are difficult to find so let's pretend that our SAE 40 oil has a viscosity = 14.4 cSt @ 100°C & VI = 75. We would reach the upper viscosity range (12.5 cSt) for SAE 30 with our SAE 40 oil at about 105°C (221°F) and the lower range (9.3 cSt) at about.117°C (243°F). In other words, modern 30-grade oils (with their higher viscosity indexes) can perform better in extremely hot conditions than the 40-grade oils available when the car was built.

At the other end of the temperature range, the 1938 Dodge required 10W oil at temperatures above 10°F (-12.2°C). Again, oil specifications for 1938-era oils are difficult to find so let's pretend that our SAE 10W oil has a viscosity = 4.1 cSt @ 100°C & VI = 75. We would reach the upper viscosity range (12.5 cSt) for SAE 30 with our 10W oil at about 55.5°C (132°F) in the sump. A modern 10W-30 oil would obviously be able to maintain its viscosity right up to its full operating temperature.

Modern Oil Use

We can't find the recommended oils for older vehicles any longer. Just as well, because even the worst modern oils have much better viscosity indexes and additive packages than the best oils of the years the old cars were made. Using LUBRIPLATE Non-Detergent Motor Oils as an example of a non-detergent API SA engine oil, we can see that the SAE 30 grade has a VI of 109 which would be a superior oil on the early VI scale.

However, a major issue for cars since the 1930s has been sludge and modern oils combat this problem with a variety of additives including detergents and dispersants. Detergents keep oil-insoluble contaminants in suspension and dispersants keep contaminants from agglomerating (i.e., remaining in a dispersed state).

Another issue, especially with modified engines having aftermarket high performance valve trains, is the protection of flat tappets. The sliding friction occurring as the valve lifter moves over the cam lobes can have excessive wear with high rate springs. The anti-wear additive commonly used for this protection is ZDDP (Zinc dialkyldithiophosphate), which has been reduced in the latest SN engine oils because its post-combustion ashes can poison catalytic converters and O2 sensors. The phosphorus component of ZDDP is what actually performs the anti-wear function.

Since many of the cars owned by ACCCC members generally do not have concerns with catalytic converters and since we would prefer all the anti-wear protection available in a modern engine oil, the best oils for flat tappet engines are the HDEOs used in diesel engines. CI-4 and CI-4+ HDEOs have the highest levels of ZDDP and and, because of nature of diesel combustion, also have the among best engine cleanliness additives. However, these engine oils are commonly found with 15W-40 viscosity, which is unnecessarily heavy for use in automotive engines. Higher than necessary viscosity results in increased engine friction which results in poorer fuel economy and wasted power.

HDEOs are also available with lower winter (W) ratings but are these are generally synthetic based. Commonly available HDEO viscosities include 5W-40 and 0W-40 in addition to 15W-40. 0W-30 HDEOs are also available from a few manufacturers. Although synthetic oils are generally about twice cost of conventional motor oils, it is possible to extend the oil change intervals (OCIs) significantly. Besides extended OCIs, synthetics have extremely low pour points and, therefore, flow readily at extremely low temperatures.

See the following Mobil videos for more information about the capabilities of synthetic engine oils:

Oil Change Intervals

Many people feel that 3000 mile (5000 km) OCIs are necessary. This notion may be a hold-over from early maintenance practices and is definitely reinforced by modern marketing. Obviously, an oil change shop will make more money if you bring your car back to them more often. Even in 1965, the OCI recommended by the factory for Plymouth Valiant normal service was 4000 miles (6400 km) and this was with a API MS/SC engine oil. Modern oils, especially synthetics, can easily go much more than 5000 miles (8000 km) between changes with non-severe driving. To save money on unnecessary oil changes and wasted time, use a synthetic oil and time your oil changes to coincide with tire rotations. Tire rotations are commonly done every 10,000 km or every 5000-6000 miles. Newer cars equipped with oil life monitoring features such as the GM Oil Life Monitor (OLM) can safely drive their vehicles until the OLM indicates an oil change is due.

For those people using conventional mineral engine oil, the recommendations found in your owner's manual should be followed. Although many oil manufacturers tout their products' ability to have extended OCIs, it probably makes the most sense to time your oil changes to coincide with other scheduled maintenance so as to minimize the vehicle's downtime as well as your time.

Used Oil Analysis

The only way to truly know whether an engine oil has reached the end of its lifespan is to have a sample of it analysed by an oil testing lab. It is normal for an oil to become dark because the detergent/dispersants are keeping contaminants in suspension. Oil analysis will indicate whether the additive package has been depleted and measure the contaminant load in the oil. If you want to significantly extend your OCIs, please consider using this service. See What is Oil Analysis? for more information.

General Recommendations

Pretty much any modern oil API Starburst or SJ, SH, SL, SM, or SN rating will work fine in unmodified automotive gasoline engines. Although your engine may have called for an API MM, MS, SB, SC, SD, SE oil, you can use the latest SN oil because the ratings are backwards compatible. Potentially, SN/SM Starburst oils may not have enough ZDDP for aggressive aftermarket valve trains. CI-4+/SL oils would likely have all of the ZDDP required for demanding flat tappet cam applications. In either case, the important thing is to use an oil with the same hot-viscosity rating as recommended by the manufacturer. That is, if your owner's manual called for a SAE 30 or a SAE 10W-30 oil, you can also safely use a 0W-30 or 5W-30 because ALL 30-weight oils must have their hot (100°C / 212°F) viscosity fall within the range of 9.3 cSt to 12.5 cSt. The first part (W-grade) of an SAE multi-viscosity oil (e.g., SAE 0W-30., 5W-30, 10W-30) just describes the oil's cold-flow characteristics. The lower the W-grade, the better the oil flows in cold weather.

Bearings depend on oil FLOW rather than oil PRESSURE for lubrication. Since oil pumps are positive displacement devices, flow is directly proportional to RPM and pressure is a characteristic of flow resistance. Higher viscosity oils like a 10W-40 or a 20W-50 can cause your oil pump's relief valve to bypass oil flow back to the sump unless your engine oil temperature is hot enough to reduce the heavier oil's viscosity enough to keep the developed pressure below the relief valve's bypass setting. If your oil pressure rises to a maximum level and stays there continuously, your oil pump may be in continuous bypass mode.

Wear is minimized in automotive engines when the engine oil viscosity is within its design range. An engine will wear faster when the oil is too thin or too thick so viscosity must therefore be in the right range for maximum engine life. Excessive thickness is especially an issue at low temperatures where a viscous oil's resistance to flow could cause cavitation at the oil pump, air binding failure, or flow limited failure. (See Submergence level and vortex formation to see how air is sucked into the pump suction inlet.) A small amount of entrained air can cause a significant adverse affect on pump performance. In either case of air binding or flow limited failure (i.e., if your oil pressure takes longer to come up than what is normal in warm weather), it is important to switch to a lower viscosity oil. Excessive viscosity also unnecessarily stresses oil pumps, which could cause premature wear of the drive gear. Although most collector car owners do not drive their collector vehicles in the winter, the low temperature flow capability of a 0W or 5W oil allows it to flow to the bearings faster than heavier weight oils at start-up. Using an excessively heavy oil will cause the safety relief valve to bypass oil back to sump at cold start-ups, thereby reducing oil flow to the bearings. Winter-driven daily driver vehicles would definitely benefit from low viscosity 0W/5W multi-grade oils year-round.

I don't recommend the use of racing oils (oils with high levels of ZDDP) in street-driven vehicles due to their generally lower levels of detergency. However, engines with highly stressed valve trains may benefit from racing oils if short OCIs are used to minimize sludge formation. However, HDEOs often contain the same or greater amounts of ZDDP but with much better detergents at a lower cost. Esso also recommends the use of HDEOs for flat tappet engines (see Esso's Flat Tappet Engine Wear Bulletin for more information).

Engine cleanliness has always been of great importance for a well-maintained car. Sludge became a major problem during the winter of 1935 and road-draught crankcase ventilation systems were introduced as a result. By 1962, Positive Crankcase Ventilation (PCV) systems were introduced for emissions and improved sludge control. You can help minimize engine contaminants with a few simple things:

  • Ensure the crankcase ventilation system in good working order.
  • Ensure that the crankcase ventilation filter is clean and in good condition. For road-draught and open PCV systems, consider using a high efficiency filter in place of the standard oil filler cap filter while driving to minimize the amount of dust from entering the crankcase.
  • Ensure that the manifold heat control system is in good working order. This allows faster warm-ups and minimizes any liquid fuel from entering the crankcase. Remember: it is actually the intake manifold that is warmed-up when your engine is warmed-up (ie, running smoothly with the choke off).
  • Drive the car regularly long enough to allow the engine oil to reach operating temperature. This helps to drive off condensation and minimizes fuel dilution.
  • Use a good quality paper element air filter cartridge if so equipped. Low restriction filters generally do not filter as well as OEM-style filters.

Oil Recommendation

For both newer and older engines in good condition requiring a 30-weight oil, I recommend the use of a 0W-30 or 5W-30 CI-4/SL or CJ-4/SM HDEOs in place of SAE 30 and 10W-30. However, 5W-30 and 10W-30 HDEOs would still work well in older vehicles requiring a 30-grade oil. Oil burners (ie, gasoline engines that consume a lot of engine oil) should stick with 10W-30 HDEOs for its lower operating cost. The only 0W-30 HDEOs I've been able to locate so far are listed in the Heavy Duty Engine Oils article. Because 0W-30 has a lower viscosity than 10W-40 or 20W-50-oils that many people like to use, a 0W-30 oil will have less internal friction and will have better heat transfer through the engine thereby carrying away more heat to the oil pan and filter for cooling. In other words, lower viscosity oils tend to run cooler. Because the 0W-30s are synthetic (or semi-synthetic) oils, they also have extremely low pour points that even suitable for arctic winter operation and there would be no need to change to a lower viscosity oil for cold weather use.

Dual API ratings (like CJ-4/SM) means that these oils are suitable for both gasoline and diesel engines at the expense of potential slight fuel economy savings (due to the lack of friction modifier additives). CJ-4/SM oils are safe for use in modern vehicles equipped with catalytic converters requiring 10W-30 or 5W-30. You may also safely use this in air-cooled engines requiring SAE 30 engine oil. Engines requiring a heavier weight oil such as 15W-40 may safely use a 0W-40 oil for improved fuel economy and reduced start-up wear. Engines with aftermarket high rate valve springs and high lift camshafts will benefit from the higher ZDDP levels in CI-4 oils but these are getting harder to find. However, CJ-4 oils generally have backwards compatibility with CI-4+, CI-4, and CH-4 oils. However, engines from the 1960s and 1970s only required API SC to SE so modern oils would still offer more protection than originally specified by the OEM.

Synthetic 0W-30 HDEOs are more expensive than conventional oils. If price is an issue or if 0W-30 oils are not readily available in your location, many engine oil manufacturers also make 10W-30 HDEOs which should work similarly well as 0W-30 oils in gasoline-powered vehicles in warm weather. For summer-time use, older vehicles often were recommended to use 20W-40. In this case, a 15W-40 would be an excellent (and very economical) substitute. Very thick oils like 20W-50 are generally not required in street-driven engines and will waste both power and fuel economy under normal operating conditions. Many BITOG members prefer using high-mileage oils (such as Quaker State Defy - with API SL for 10W-30) which tend to have higher levels of ZDDP than Starburst oils.


Oil Recommendation for Early Engines

For early engines, please determine what bearings with which your engine is equipped before trying new oils. It is very possible that the additives of ANY modern (as in not non-detergent SA) engine oil would have a negative impact on non-tin/lead (such as silver-lead) babbitt bearings. If the engine has tin/lead babbitt or shell insert bearings, any modern oil would likely be suitable. Shell insert bearings were introduced around 1933. For early babbitt bearing engines, castor oil may still be the best lubricant.

Another concern with older engines is PAO oil's effect on internal components made from Polyoxymethylene plastic (aka Dupont Delrin). PAO oils can potentially cause stress cracking of plastic engine parts made from this plastic. A Group III oil may be a better choice in engines having internal Delrin plastic parts.

Many Model T owners have upgraded the friction linings in their transmission bands from cotton to Kevlar. The use of Kevlar completely overcomes the problem of cotton fibre disintegration but creates a transmission slippage problem instead. To reduce Model T transmission slippage with a Kevlar upgrade, I don't recommend the use of Starburst oils as these contain friction modifiers that can exacerbate transmission slippage. Motorcycle or HDEO oils suitable for wet-clutch applications may be a better oil in Model T applications. Another oil potentially suitable for Model T transmissions with a Kevlar upgrade is an HDEO similar to Mobil Special 30 which would provide better sludge and wear protection than a non-detergent oil like LUBRIPLATE Non-Detergent Motor oil. In colder weather, a Type F automatic transmission fluid may provide better slip protection but does not offer any detergency with a resulting risk of sludge. The viscosity of a Type F ATF would be similar to a 20-weight oil (see Mobil Type F ATF as an example).