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In 1971, the S-categories were introduced which superseded the M-categories.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.