The API Service Symbol “Donut” is divided into three parts:
The top of the Donut shows the oil’s performance level for gasoline and/or diesel engines. The letter “S” followed by another letter (for example, SM) refers to oil suitable for gasoline engines. The letter “C” followed by another letter and/or number (for example, CJ-4) refers to oil suitable for diesel engines. These letters officially stand for “Service” and “Commercial.” The current API performance categories that can appear in the top part of the Donut are listed in the API Service Category Chart.
The center of the Donut shows the oil’s SAE viscosity grade. Viscosity is a measure of an oil’s flow characteristics, or thickness, at certain temperatures.
The low-temperature viscosity (the first number, 5W in a 5W-30 oil) indicates how quickly an engine will crank in winter and how well the oil will flow to lubricate critical engine parts at low temperatures. The lower the number the more easily the engine will start in cold weather.
The high-temperature viscosity (the second number, 30 in a 5W-30 oil) provides thickness, or body, for good lubrication at operating temperatures.
A multigrade oil (for example, SAE 5W-30) provides good flow capability for cold weather but still retains thickness for high-temperature lubrication.
A single grade oil (a single number in the center of the donut) is recommended for use under a much narrower set of temperature conditions than multigrade oils.
Operators should refer to their owner’s manuals to select the proper viscosity oil for the ambient temperature and operating conditions at which the equipment will be used.
The bottom of the donut tells whether the oil has energy conserving properties when compared with a reference oil in an engine test. Oils labeled as “Energy Conserving” have passed this test. Widespread use of engine oils with this designation should result in an overall saving of fuel in the vehicle fleet as a whole, but a particular vehicle operator may not experience a fuel savings as a result of using these oils.