American Petroleum Institute (API) is the national trade association that represents all aspects of America’s oil and natural gas industry. API provides the API’s Engine Oil Licensing and Certification System (EOLCS). A voluntary licensing and certification program that authorizes engine oil marketers who meet specified requirements to use the API Engine Oil Quality Marks. The American Petroleum Institute (API) classification system provides information about the engine oil.

Letter “S” stands for “spark” and followed by another letter (for example “SM”) indicates that the oil is appropriate for use in petrol engines or gas engines. SM oils are superior to SL oils in many performance criteria including enhanced fuel efficiency, also meeting the “energy conserving” classification and enhanced emission systems protection.

Letter “C” comes from the word “compression” and followed by another letter and/or number (for example CI-4) indicates that the oil is appropriate for diesel engines. The second letter in both the “S” and the “C” categories is assigned alphabetically. Generally the further along we move in the alphabet indicates the latest technology and quality enhancements in motor oil.

S series:
– SA – category until 1930
– SB – category until 1951
– SC – category until 1967
– SD – category until 1971
– SE – category until 1979
– SF – category until 1988
– SG – category until 1993
– SH – category until 1996
– SJ – category starting with end of 1996
– SL – category starting with end of 2001
– SM – introduced in 30 November 2004
– SN – introduced in 2010

As this classification evolves the oils belonging to the new generations meet the requirements of the old cars. Thus a API SL oil meets the requirements of a car manufactured in 1965.

C series:
– CA – category between 1940-1950
– CB – category between 1949-1960
– CC – introduced in 1951
– CD – introduced in 1955
– CD-II – introduced in 1987 for 2 stroke engines
– CE – introduced in 1987
– CF – category since 1994
– CF-2 – category since 1994 for 2 stroke engines
– CF-4 – category since 1994 for 4 stroke engines
– CG-4 – category since 1996
– CH-4 – category since 1998
– CI-4 – category since 2002
– CJ-4 – category since 2006

API classification subdivides all transmission, transfer cases and differentials oils into 6 classes:

GL series:
– GL-1 – oils for light conditions
– GL-2 – oils for moderate conditions
– GL-3 – oils for moderate conditions
– GL-4 – oils for various conditions
– GL-5 – oils for severe conditions
– GL-6 – oils for very heavy conditions