American Petroleum Institute (API):

API Classifies oils according to their capacity to protect the engine. The letter S is used for petrol engines and the letter “C” for diesel. The letter that follows the “S” or the “C” indicates the degree of performance and protection, with “A” the lowest and “X” highest. Examples of API are CC,CD,CF-4,CH,CH-4,CI,CI-4,SC,SF,SG,SH,SL,SM ,SN etc. Some oils can be used both in petrol and diesel engines, so their classification is stated as SJ/CF or CF-4/SJ.

Society of Automotive Engineers (SAE):

The SAE is the Society of Automotive Engineers. The SAE classifies motor oils according to certain viscosities and very general temperature ranges at which they can be used. Automobile and equipment manufacturers also specify which oil should be used for a particular ambient temperature operation. It is important to note that the viscosity and temperature range values listed by automobile and equipment manufacturers are almost always meant for petroleum oil. Oil can be mono-grade or multi-grade, depending on its viscosity performance at various temperatures. Examples of SAE are 40, 50, 20w-50, 15w-40, 10w-40, 5w-30 etc.

ACEA Classification (Association des Constructeurs Européens d’Automobiles):

The European Automobile Manufacturers Association (ACEA), founded in 1991, represents the interests of the sixteen European cars, truck and bus manufacturers. Similar to the API,
with the difference that while the API is based on American Engines, the ACEA is based on European engines. The ACEA also uses letters to denote classifications.
ACEA differentiates three engine oil categories:
Otto-engine cars: A
Diesel-powered cars and vans: B
Diesel-powered commercial vehicles: E