The " API Donut "

The symbols give you three pieces of information.

API (American Petroleum Institute) Service Rating - this two letter classification identifies the vehicle fuel type and quality level of the motor oil. The first letter indicates the vehicle fuel type that the oil is designed for. Ratings that begin with an "S" are intended for gasoline engines. Ratings that begin with a "C" are for diesel engines.
The second letter designates the quality level of the motor oil. The higher the letter, the more advanced the oil and the more protection it offers your engine. An SJ oil can be used in any engine requiring an SB, SG, SH, etc. oil.

The latest service category rating for gasoline engines is "SM", introduced in November 2004 for 2005 and newer engines. SM-rated oils, also previous "SL" (2001) and "SJ" (1997) ratings, are backwards compatible and can be safely used in older engines. Older obsolete service classifications (SH, SG, SF, etc.) may not meet OEM lubrication requirements for newer engines. Likewise, API SL oils should not be used in 2005 and later vehicles, and SJ oils should not be used in 2001 and newer vehicles.

For diesel engines, API has a separate rating system. The current category is "CI-4" (introduced in 2002 for newer diesels that have exhaust gas recirculation). The previous CH-4 (1998), CG-4 (1995), and CF-4 (1990), can all be used in older four-stroke diesel engines. CF-2 (1994) is the API classification for two-stroke diesels.

API also gives oils an "Energy Conserving" rating if the oil meets certain criteria for reducing friction and oil consumption, and improving fuel economy. The API Certification Mark "Starburst" is designed to identify engine oils recommended for a specific application (such as gasoline service). An oil may be licensed to display the Starburst only if the oil satisfies the most current requirements of the International Lubricant Standardization and Approval Committee (ILSAC) minimum performance standard for this application (currently GF-4 for passenger cars). Many automobile manufacturers recommend oils that carry the API Certification Mark.

Motor oils that meet the current API SM rating may also meet the new International Lubricant Standardization and Approval Committee (ILSAC) "GF-4" specifications, which some European and Asian auto makers require.