Engine Oil Grades and Classifications

Using the proper engine oil is important for a car or truck engine to get the maximum mileage possible. Using the wrong engine oil can lead to premature engine wear.

Every can or bottle of engine oil that is used in a vehicle’s engine should have an American Petroleum Institute (API) service symbol listed. The API and vehicle manufacturers got together to classify and certify motor oil to meet the vehicle manufacturers’ standards. Using the proper engine oil classifications is important to ensure that the vehicles engines run at peak performance.

The API Motor Oil Service Symbol

The API service symbol is broken up into three parts. The symbol is round and is often referred to as the donut.

• The top half of the donut indicates the performance level of the oil.
• The bottom half of the donut indicates if the oil conserves energy.
• The center of the donut indicates the SAE oils viscosity rating.

API Motor Oil Performance Level

For car and truck motor oil the performance level usually has at least two letters. The first letter is either an “S” or a “C”. The S indicates the oil is for gasoline engines and stands for “Service”. The C indicates the oil is for diesel engines and stands for “Commercial”.

The second letter indicates the service level of the motor oil. With new and improved oil, one letter will usually bump up the service letter. The current level for gasoline engines is SM, assigned in 2004, and is suitable for older vehicles. For example, if a 2003 vehicle owner’s manual requires a performance level SJ, an SM performance level rating is perfectly acceptable.

The current level for diesel engines is CJ-4 (4 means four-stroke diesel engines), assigned in 2006. The same rule may not apply for diesel engines as it does with gasoline engines from one model year to the next. For more information refer to the API Engine Oil Guide in PDF format.

API Listing for Motor Oil Energy Conservation

If the API Donut states “Energy Conserving” at the bottom it indicates that the motor oil has passed various tests to conserve fuel. The actual amount of fuel that it may conserve may not be substantial enough to notice much of a difference.

SAE Motor Oil Viscosity Rating

The middle of the donut indicates the SAE (Society of Automotive Engineers) Viscosity Rating. Viscosity is the measurement of the oil’s resistance to flow. For example, water would have a very low resistance to flow and honey would have a high resistance to flow.

Today’s vehicle motor oils have two viscosity ratings separated by a “W”, for example, 5W30. The first number(s) before the W is the viscosity ratings for cold weather, especially important for cold weather engine starting. The numbers after the W indicates the viscosity ratings for hot temperatures after the vehicle reaches operating temperatures.

API Motor Oil Certification Mark

Look for the API Certification Mark next to the API Service Symbol on the label of the oil. The mark will be in the form of a starburst with the words “American Petroleum Institute Certified”. The center of the starburst will indicate either for gasoline or diesel engines.

This symbol or mark indicates that the oil is API certified in accordance with certain vehicle manufacturers and the lubrication industry. Many vehicle manufacturers recommend engine oil with this certification mark. This ensures the quality and integrity of the motor oil. For a better performing oil, synthetic oil may also be a consideration.

It’s important that the correct oil is used in a vehicle’s engine for performance and engine longevity. The vehicle owner’s manual should list the correct API performance rating and oil SAE viscosity rating for the vehicle’s engine. The next time the vehicle engine oil gets changed, make sure the oil used is the correct performance level and the correct viscosity rating.