The American Petroleum Institute rates motor oils in the U.S., through a system for determining a motor oil's proper use based upon its viscosity and energy conservation qualities.
Look on the back of any bottle of motor oil to see if it meets OEM quality and performance standards set by the API. If it does, it will display the API service symbol, sometimes called the "API donut," which shows the oil's latest service category and viscosity ratings and whether it rates as an energy conserving oil.
The service category refers to the range of cars that can use a given oil, determined by the vehicle's date of manufacture. Recent service categories include SN for 2011, SM for 2005-2010 and SL for 2001-2004.
Oil viscosity measures the thickness of a motor oil at a given temperature. Referred to as the weight of the oil, viscosity is designated as a "W" after the weight value, such as 20W. Multi-grade oils -- 20W-50, for example -- contain two weight numbers in their rating.
If an oil ranks as energy efficient, the API service symbol will include the words "Energy Conserving."
