The Api (American Petroleum Institute) Engine Oil Classification System Was Set up as a Joint Effort by Api, Astm (American Society for Testing Materials) and Sae (Society of Automotive Engineers).
The Api Engine Classifications Are Divided into Two Major Categories Indicated by the First Letter.
(1) The ‘s’ Series Designation Stands for Service Station, and Is Used to Designate Products for Use in Gasoline Engines.
(2) The ‘c’ Series Designation Stands for Commercial, and Is Used to Designate Oils for Use in Diesel Engines.
The Second Letter Indicates the Performance Capabilities of Engine Oils as Defined for Series of Engine Tests That Must Be Passed Periodically, as More Severe Engine Requirements Emerge, New Api Service Classification Designations Are Added Proceeding Further into the Alphabet.
The Api Classification Should Not Be Used as a Strict Measure of Engine Oil Quality but Rather as a Guide to Its Application.