Abbreviation for the American Petroleum Institute. API is the only national trade association that represents all aspects of America’s oil and natural gas industry.

For consumers, API provides the API’s Engine Oil Licensing and Certification System (EOLCS). It is a voluntary licensing and certification program that authorizes engine oil marketers who meet specified requirements to use the API Engine Oil Quality Marks. These emblems go directly on each container of oil and is there to help consumers identify quality engine oils for their gasoline- and diesel-powered applications.
See: http://www.api.org/