API Engine Oil Classifications
The American Petroleum Institute (API) has established a classification system for the designation of gasoline and diesel engine oils which reflects the oils quality, performance and suitability for various engines. These classifications, or categories as sometimes referred, have no bearing on oil viscosity, whose limits are set by the Society of Automotive Engineers (SAE) and designated in SAE J300. Contrary to the belief of many, SAE grade only defines the oil's viscosity and has absolutely nothing to do with oil quality. To specify an engine oil, both the API service designation and the SAE viscosity grade are required.
In the United States, the API also administers the licensing and certification of engine oils through a classification system that reflects the warranty, maintenance and lubrication requirements of the automotive industry. Through this system, API has
standardized the --labeling of engine oils by adopting the "donut logo", which tells the user the oil's viscosity grade, engine service classification and any energy conserving capabilities. Engine oil performance requirements, test methods and limits for the various classifications are established by the engine and vehicle manufacturers and technical societies.
These classifications are arranged into two different groups, one for automotive gasoline engine service and one for commercial diesel engine service. The former is listed in "S", or "service" oils (presently SA through SH) and the later is listed in "C", or "commercial" oil (presently CA through CG-4) classifications. The higher the second letter, alphabetically, in the designation, the higher quality and performance offered by that oil.