You've seen 5W-30 or 10W-40 on oil containers. What exactly do these numbers mean, and how do you pick the right one?

Breaking Down the Numbers

The viscosity grade (SAE grade) describes how the oil behaves at different temperatures:

  • Number before "W" (e.g. 5W or 10W): "W" stands for Winter. The lower this number, the more fluid the oil is in cold conditions — meaning it reaches engine parts faster on a cold start.
  • Number after "W" (e.g. 30 or 40): Describes the oil's stability at normal operating temperature. A higher number means thicker oil that maintains its film strength in heat.

Practical Examples

  • 5W-30: Flows well in cold + stable in heat. The most common grade for modern engines.
  • 10W-40: Slightly thicker, suited for engines with higher oil consumption or very hot climates.
  • 15W-40: Traditional grade for older engines and diesel commercial vehicles.

Which Grade Is Right for Me?

The definitive answer: whatever your owner's manual specifies. Using the wrong grade can increase oil consumption, reduce efficiency, or damage engine components over time.