Two important specifications found on oil bottle labels are the Society of Automotive Engineers viscosity grade and the American Petroleum Institute service classification. These specifications ensure that the oil meets the quality and performance standards set out by the original equipment manufacturer.
The SAE numerical system for grading oil viscosity uses the numbers 0, 5, 10, 15 and 25. It includes the letter W if the engine oil has a winter grade designation.
The API service classification is displayed on oil bottles as a two-letter code. The first letter splits oil specifications for passenger cars into two categories: S for gasoline engines and C for diesel engines. The second letter correlates to the model years that oil is formulated for.