The first oil classification according to its application and operating features was proposed by the American Petroleum Institute (API) far back in 1947.
This classification has being upgraded ever since, however retaining the current principle of oil division in two categories, "S" and "C". The "S" category comprises oils for four stroke petrol engines, whereas the "C" category comprises oils for diesel engines, cars, trucks, construction equipment and agricultural machinery.
Due to the ever growing oil quality requirements, the operating features grade of the "S" category according to API is divided in 10 classes (SA, SB, SC, SD, SE, SF, SG, SH, SJ, SL), whereas that of the "C" category is divided in 11 classes (CA, CB, CC, CD, CD-II, CE, CF, CF-2, CF-4, CG-4, CH-4).
Designations to CD-II, CF-2, CF-4, CG-4 and CH-4 classes provide further information on how to use such oils with 2-stroke and 4-stroke diesel engines.