API donut
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).
API donut: the motor oil label that tells you what you're buying
The API donut is a standardized label printed on motor oil containers in North America. It consists of a circular design divided into three zones: a center circle showing viscosity grade (like 10W-40), an upper segment naming the oil type and performance level (like SP or SN Plus), and a lower segment listing additional certifications. The American Petroleum Institute created this format to give mechanics and consumers instant clarity on whether an oil meets their engine's requirements.
The viscosity numbers in the center are the critical specification. The first number before the W (winter) indicates cold-start flow at minus 18 degrees Celsius; the second number after it indicates flow at 100 degrees Celsius during running temperature. A 10W-40 oil flows like a grade 10 in cold conditions but maintains the body of a grade 40 when hot. Thinner oils (lower numbers) reduce friction and improve fuel economy; thicker oils (higher numbers) maintain film strength under heat and load. The W rating matters most in cold climates; the hot rating matters most for engines running in high-temperature or high-stress conditions.
Reading and applying the API donut
The top segment shows the API Service Category, which changed significantly in 2020 when the SP designation replaced SN. This category reflects the oil's detergency, oxidation resistance, wear protection, and deposit control relative to the latest engine designs. Older engines often cannot use the newest oils, so matching the donut label to the vehicle manual is essential. Many containers also include ACEA or JASO ratings for European or Japanese specifications, appearing in that lower segment alongside viscosity performance claims.
The donut's presence signals that the oil has been tested and certified by the API; absence of one indicates either an aftermarket blend that did not seek certification or a synthetic that met alternative standards. Counterfeit and repackaged oils sometimes bear fake donuts, so verification of the container's seal and the brand's genuine appearance matters. The label is legally required on motor oil sold in the United States and remains the fastest way to confirm an oil will not damage an engine or void a warranty.