What Do Motor Oil Numbers Mean?

Motor oil labels such as 0W-20 or 5W-30 describe viscosity behavior under standardized test conditions. They do not rank oil from best to worst.

The first number and W

The number before W relates to low-temperature performance. A lower winter grade is designed to meet pumping and cranking requirements at colder test temperatures.

The second number

The number after the dash describes viscosity requirements at operating-temperature test conditions. A 30-grade oil is thicker under those tests than a 20-grade oil.

Viscosity is only one requirement

Modern engines can also require a specific API or ILSAC performance category, manufacturer approval, or oil chemistry. Matching the viscosity number alone is not enough.

Use the owner's manual

Follow the grade and specification listed for your engine and climate. Changing viscosity to solve a noise or oil-consumption problem without understanding the cause can create new problems.

Quick checklist

Use the product label, manufacturer directions, and the specific requirements of your situation. Compare the complete product or system rather than making a decision from one marketing phrase, ingredient, or number.