The American Petroleum Institute (API) sets minimum requirement of performance standards for lubricants. Engine oil which has been tested by API would display the API Service Symbol (the "Donut) and Certification Mark "Starburst". Performance requirements, test methods, and limits are cooperatively established by vehicle and engine manufacturers, including Society of Automotive Engineers (SAE), the American Society for Testing and Materials (ASTM), American Chemistry Council and American Petroleum Institute (API).
The API Service Symbol
The API Service Symbol "Donut" is divided into three parts:
The bottom of the donut tells whether the oil has energy conserving properties when compared with a reference oil in an engine test or if an oil meets CI-4 PLUS requirements.
Oils labelled as "Energy Conserving" have passed the test that measures an oil's ability to conserve energy. 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.
Used in conjunction with the CI-4 category, the "CI-4 PLUS" designation identifies oils formulated to provide a higher level of protection against soot-related viscosity increase and viscosity loss due to shear in diesel engines.
The top of the Donut shows the oil's performance level for gasoline/petrol and/or diesel engines. The letter "S" followed by another letter (for example, SN) refers to oil suitable for gasoline/petrol engines. The letter "C" followed by another letter and/or number (for example, CI-4) refers to oil suitable for diesel engines. These letters officially stand for "Service" and "Commercia
SAE Viscosity Grade
The centre of the Donut shows the oil's SAE viscosity grade. Viscosity is a measure of oil's flow characteristics, or thickness, at certain temperatures. The low-temperature viscosity (the first number, 5W in 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 multi grade 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 centre of the donut) is recommended for use under a much narrower set of temperature conditions than multi grade 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 API Certification Mark
The API Certification Mark "Starburst" is designed to identify engine oils recommended for a specific application (such as gasoline service). An oil may be licensed to display the Starburst only if the oil satisfies the most current requirements of the International Lubricant Standardization and Approval Committee (ILSAC) for the minimum performance standard for this application (currently GF-5 for passenger cars). Many automobile manufacturers recommend oils that carry the API Certification Mark.