The story
From API + donut; from American Petroleum Institute, and its shape.
Etymology from Wiktionary, CC BY-SA 4.0.
API donut
API do·nutToday it means: An information table found on cans of motor oil, conforming to U.S. standards, which indicates the weight (viscosity) and type of oil. The central circle provides the oil viscosity rating (ie. 10W50), while the upper annulus section provides the oil type (ie. SP). Full definition →