What Do Oil Numbers Mean?

Two letters that stand alone on the oil can label represent the engine type and the detergent additive. These are codes such as SE or CD, for example. S indicates the oil is for gasoline engines and the C indicates it's for diesel. The second letter indicates the level of cleansing protection. They are ordered alphabetically from worst to best. For example, F is better than E is better than D. Don't confuse "SE" with "SAE," which you may also see on the can. SAE stands for the Society of Automotive Engineers. They are mentioned on oil cans because they standardize the oil viscosity scale to which the other numbers on the can refer.

Single or compound numbers convey the oil's weight or viscosity; 30 and 10W-30 are examples. With the introduction of new oil additives starting in the 1940s, the compound viscosities replaced single-viscosity motor oil. The "W" refers to "winter." The first number is the viscosity when the oil is cold. The number after the "W" is the viscosity after being warmed by the engine to operating temperature.

Higher viscosity numbers mean a greater resistance to flow and greater friction between the oil and the parts it lubricates. The SAE scale ranges from 5 up to 50, with 50 being the thickest. The optimal viscosity depends on the operating temperature and the engine structure. Viscosity needs even vary within a single car trip.

Viscosity normally decreases as temperature increases. The hydrocarbon molecules vibrate too fast at higher temperatures to stick together as much as at lower temperatures. The balancing trick for a motor oil is not to be so viscous, or thick, that auto parts can't slide past each other when the weather is cold or the engine is just warming up but also to be thick enough not to boil away at operating temperatures in warm or hot weather.

Oil manufacturers add certain carbon polymers to motor oil to counter the inverse relationship between thickness and temperature. These "multi-weight" oils have the compound numbers with the "W" on the can. Before such additives were commercially widespread in the 1950s, oil cans had only one number for the viscosity. You would have to change your oil for the change of seasons. Drivers used low viscosity oil in the winter and high viscosity oil in the summer. By contrast, multi-weight oil can behave, say, like an SAE 10 oil in winter or when the engine first starts and an SAE 30 oil at normal operating temperatures. This allows proper lubrication of the engine through the whole range of temperatures in a single trip and in a whole year.