API Classifications in Gas EnginesService category ratings are listed with a two-letter code beginning with S for gas engines and C for diesel engines. These ratings refer to the quality level of the motor oil. SM is the highest rating, and oil of this rating is used in the newest car engines. Lower-letter combinations, such as SG, SF and SE designate compatibility only with older engines. The warranty requirements for specific engine years determine the quality of motor oil that engine requires. The American Petroleum Institute considers all classifications of SH and below to be obsolete, and recommends SJ for 2001 and older engines, SL for 2004 and older engines and SM for all newer engines. SM and SL-rated motor oils are considered backward-compatible with older engines.
API Classifications for Diesel Motor OilDiesel oil quality is denoted with a two letter code beginning with C. The American Petroleum Institute considers motor oil classed at CE or below to be obsolete. Current diesel motor oils are more varied and specialized than gas engine oils, and can be applied as follows:
CF: For use in off-road, indirect-injected and other diesel engines including those using fuel with more than 0.5 percent weight sulfur. First introduced in 1994, this oil can be used in place of older CD oils.
CF-2: For use in severe duty, two-stroke cycle engines. Also introduced in 1994, it can be used in place of older CD-II oils.
CF-4: First introduced in 1990, this oil is used in high-speed, four-stroke, naturally aspirated and turbocharged engines. It can also be used in place of older CD and CE oils.
CG-4: Used in severe duty, high-speed, four-stroke engines using fuel with less than 0.5 percent weight sulfur. This oil is required for cars that meet 1994 emission standards, and is backward-compatible with CD, CE and CF-4 oils.
CH-4: This oil was designed to meet 1998 exhaust emission standards and is backward-compatible with CD, CE, CF-4 and CG-4 oils.
CI-4: This oil meets 2004 exhaust emission standards and can be substituted for CD, CE, CF-4, CG-4 and CH-4 oils.
CI-4 PLUS: This designation identifies oils that provide the highest protection from soot-related viscosity increase or loss due to shear in diesel engines.