A Lesson In Oil

Jobbers are not technicians, nor are they scientists, but most have a pretty good idea of their technicians’ needs. Technicians rarely deviate from their trusted brand, and when it comes to purchasing wholesale, they will often go with the most common grades (most likely 10W-30 or 5W-30), ensuring the widest coverage for the vehicles their customers are maintaining.

But what do oil manufacturers mean when they say 10W-30 as opposed to, say, 5W-30?

All grades of oil are based on a viscosity range at a standard temperature set by the Society of Automotive Engineers (SAE). These numbers are referred to as the “weight” of the oil.

However, the letter “W” on an oil grade refers to “winter” (not “weight”), which is gauged by the oil’s viscosity at 0^oF. Warm-weather weight, which includes grades with no “W” in them, is based on the viscosity measured at 210^oF.

SAE viscosity gradings include the following, from low to high viscosity: 0, 5, 10, 15, 20, 25, 30, 40, 50 or 60, yet only six of them are suffixed with the letter “W” (0W, 5W, 10W, 15W, 20W, 25W respectively).

The most common multigrade monikers–indicated by two grade numbers–are 5W-30 for colder climates (falling below 0^oF), 10W-30 for intermediate climates (down to 0^oF), and 20W-50 for warmer climates (down to 32^oF).

High-molecular-weight polymers are added to a low-viscosity oil base stock to create multigrade oils that work through thick and thin. At cold temperatures, the rubberlike polymer molecules exist as balled-up coils and don’t thicken the oil significantly. However, at warmer temperatures, they expand to more linear random coils to prevent oil from thinning out too much.