The API donut is a standardized symbol on engine oil packaging that certifies the fluid meets American Petroleum Institute standards. The center of the circle displays the SAE viscosity grade, the top half shows the performance service category, and the bottom half indicates if the oil has resource-conserving properties. It provides mechanics and operators a quick visual check that the correct oil is going into the sump.
Heard on the job
Check the API donut on that drum before you pump it to make sure it's actually rated SP.
Insider tip
Always check the top half of the donut for gasoline engines (S categories) versus diesel engines (C or F categories) to prevent catastrophic engine failure.