The API rating system for motor oil grades oil based upon its performance in a variety of tests that determine its suitability for certain kinds of engines. The American Petroleum Institute uses its Engine Oil Licensing and Certification System to set industry standards and aid consumers in finding the right oil for their engines.
The American Petroleum Institute runs a voluntary system of motor oil ratings in collaboration with oil, additive and automotive manufacturers. The system rates oils according to tests set by a number of technical societies, including the American Chemistry Council. These tests are intended to improve motor oil quality within the industry and to match oils with certain types of engines.
One of the key parts of the API rating is the motor oil's viscosity under different temperature conditions. The API rating of viscosity contains two numbers. The first grades the oil's viscosity at 0 degrees Fahrenheit, while the second grades it at 212 degrees Fahrenheit. Lower numbers indicate thicker oil. Smaller differences between the two numbers indicate oil that stays at the same viscosity even under drastic temperature changes. Generally, oil thins as it heats up, which can cause it to become worse at sealing all the moving parts of the engine.
API ratings also distinguish between oils designed for gasoline and diesel engine. There are a number of different standards for oils in either category. As of 2015, the most recently added rank in gasoline engines is the SN rating, introduced in 2010 for oils that improved upon the SM category with better sludge control and better protection for pistons under high temperature conditions, among other features.