I copied this from an AMSOIL page:
The API is the American Petroleum Institute. The API maintains a system of Service Classifications (“S” ratings) for gasoline engines and Commercial Classifications (“C” ratings) for diesel engines. The procedures, procedures and policies by which lubricants are rated is a constantly changing system and involves representatives from the major automotive companies, API, oil manufacturers other engineering activities. Periodically changes are made to the specifications to keep up to date with current gasoline and diesel engine technology.
There are many classifications that have become obsolete and are no longer used. For example, occasionally I receive calls from people that have purchased a new piece of equipment stating they have to use a “SE” oil. I explain to them that SE oil specifications began in 1971 and were obsolete with the new SF specifications released in 1981. Today both SE and SF are obsolete. What has most likely happened is that some technical editor or clerk has transcribed portions of a previous service or owners manual to the new manual without completely reviewing it for any errors or updates to specifications.
The specifications referred to in these classifications generally have to do with oxidation protection, high temperature engine deposits, foaming, acid formation, sludge control and rust and corrosion protection. I have listed only the most recent classifications. There are many obsolete specifications that will not be covered in this book. These specifications are constantly changing and are subject to change as new engine, emissions and vehicles and/or government requirements arise. Note that when a new specification is released it makes the previous specification obsolete, therefore when an applications specifies the obsolete lubricant you may use the lubricant with the API classification that has superceded and replaced it
“S”- SERVICE CLASSIFICATIONS FOR GASOLINE ENGINES
SH- For 1994-1996 Gasoline Engine Service (Obsolete)
Classification SH was adopted in 1992 and recommended for gasoline engines in passenger cars and light trucks starting in 1993 model year. This category supercedes the performance requirements of API SG specification for 1989-1992 models, which is now obsolete. Applications that call for an API service classification SG can use the SH specification. The specification addresses issues with deposit control, oxidation, corrosion, rust and wear and replaces.
SJ- For 1997-2001 Gasoline Engine Service (Current Spec.)
Classification SJ was adopted in 1996 and recommended for gasoline engines in passenger cars and light trucks starting in 1997 model year. Applications specifying API SH can use the newer API SJ service classification. Note that where applicable certain letters in the sequence will be skipped to prevent confusion with other standards. In this case, SI was skipped since industrial oils are currently rated according to SI classifications.
SL- For 2001-2004 Gasoline Engine Service- (Current Spec.)
Recommended for gasoline engines in passenger cars and light trucks starting in July 2001. SL oils are engineered to provide improved high temperature deposit control and lower oil consumption. Applications specifying API SJ can use the new API SL service classification. Note that some SL rated oils may also meet the latest ILSAC specification and/or qualify as energy conserving. SL is the latest specification.
SM- For 2004 and newer Gasoline Engine Service Automotive Engines (Current Spec)
This specification was introduced in Nov. 2004. SM oils are engineered to provide improved oxidation resistance, improved deposit and wear protection and improved low temperature performance. Note that some SM rated motor oils may also meet the current ILSAC specification.
