The American Petroleum Institute (API) was formed in 1919 as a standards-setting organization and is the global leader to establish, maintain, and distribute standards for the oil and gas industry. In 1947 the API published the first engine and gear oil classification according to its application and operating features. This oil classification has been upgraded ever since. These upgrades are necessary to adapt engine and gear oils to increasing demands of engines. Challenges for oil specification are e.g. reduced fuel consumption (fuel economy) and lower exhaust emissions. But downsizing of engines according to fuel economy and emission regulations can cause „new motor failure”. The so-called LSPI (Low Speed Pre Ignition), a form of anomalous combustion, should be mentioned here. All of these increased demands on oils are taken into account in new specifications and verified by engine tests.

Since 1924, the American Petroleum Institute has been a cornerstone in establishing and maintaining standards for the worldwide oil and natural gas industry.

API differentiates between engine oils for gasoline engines (Service-category, specifications are prefixes with an “S” e.g. API SP) and diesel engines engines (Commercial-category, specifications are prefixed with an “C” e.g. CK-4). When choosing engine oil for your vehicle, it is helpful to look or ask for API-licensed oil. API has made it easy to find these oils: licensed oils display the API engine oil quality mark—the API Service Symbol "Donut". This mark is part of API's Engine Oil Licensing and Certification System (EOLCS), a voluntary licensing and certification program that authorizes engine oil marketers meeting API requirements to display the API quality marks.