There are three most common organizations that classify the motor oils based on its quality: API, АСЕА (ССМС) and ILSAC.
API (American Petroleum Institute) oils classification has two oil categories: "S" (service) and "C" (commercial). “S” oils are used in gasoline engines while "C" oils in diesel engines. On the label you will always find two letters “S” and "C" with either of them coming first and the second letter of the Latin alphabet meaning the motor oil class (the farther from the first letter of the alphabet, the higher is the oil level).
Currently, API has the following classes for gasoline engines: SG (1989), SH (1993), SJ (1996), SL (2001), SM (2004), SN (2010). And for diesel engines in passenger cars, vans and commercial vehicles: CF-4 (1990), CG-4 (1995), CH-4 (1998), CI-4 and CI-4+ (2002), CJ-4 (2010). A year, indicated in brackets, means the year when a particular specification became effective.
ACEA oil classification (European Automobile Manufacturers Association), established in 1996, provides a fuller scope for motor oils application and pays more attention to their operational properties. Oils are marked with a letter (A - for gasoline engines, B and E - for diesel) and a number (the larger it is, the better oil performance is). Hyphenated is a year of specification approval or its change. These quality classes are cited below and listed according to their performance upgrade:
Oils for gasoline engines in passenger cars, minivans and vans: А1-since 1996, А2-since 1996, А3-since 1996, А4-since 1998, А5-since 2002.
Oils for diesel engines in passenger cars, minivans and vans: B1-since 1996, B2-since 1996, B3-since 1996, B4-since 1998, B5-since 2002. (Oil class featuring letter "E" is not considered here as it states for cargo vehicles).
Since 2004, ACEA introduced a new oil class "C" (C-1, C-2, C-3, C-4) suitable for use in both gasoline and diesel engines of the latest generation. "C" class oils are low-ash oils (low content of sulphated ash, sulphur and phosphorus) and are compatible with the latest versions of exhaust gas post processing systems, which are very sensitive to ash.