The American Petroleum Institute, API, is a U.S.A trade association for the oil and natural gas industry. It reunites different actors in the production, refinement, distribution, and many other activities around petroleum. Amongst their activities, they set up diverse technical standards, such as measurement methods, standards for the design of pressure vessels, etc., but also the industry standards for the energy conservation of the motor oil. There are two categories: gasoline and diesel. Category S is for “Spark Ignition”, for Gasoline, and category C is for “Compression Ignition’, for Diesel. For both, the letter S or C is followed by another letter. For example, SM or CF. The higher the letter, the more recent the technology of the oil. For recent oil, the higher specification will supersede the lower letter grade. For example, if we have oil with specification SN, it can also be used for vehicles that request API SM, SL, and SJ; it is backward compatible.
However, you have to pay attention to the year of construction of the engine. If the engine is too old, modern chemistry might not be adapted or compatible with old engine designs and materials used to make the engine. Let’s see by category how it works.
Below are all the API categories in Gasoline. It is mainly for passenger cars, as most of the cars in the US are Gasoline and not Diesel.