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.