Motor Oil Viscosity Explained: What 0W-20, 5W-30 and 15W-40 Really Mean

Every bottle of motor oil has a grade such as 5W-30 or 0W-20. Those numbers are not random. They tell you how the oil behaves when the engine is cold and when it is hot. Using the grade the engine maker recommends protects the engine and keeps fuel economy where it should be.

How to read a viscosity grade

Take 5W-30 as an example:

• 5W — the "W" stands for winter. The lower this number, the better the oil flows when cold. A 0W oil flows faster on a cold start than a 10W oil.
• 30 — the oil's thickness at normal engine operating temperature. A higher number means a thicker oil when hot.

Most engine wear happens in the first seconds after a cold start, before oil reaches every part. That is why cold-flow matters so much in winter.

Common grades and where they are used

• 0W-20 and 0W-16 — many newer cars and hybrids designed for fuel economy.
• 5W-20 and 5W-30 — the most common grades for cars, SUVs and light trucks.
• 10W-30 — some older vehicles and small engines.
• 15W-40 — heavy-duty diesel engines, diesel pickups and equipment.
• 5W-40 synthetic — many diesel and European engines, especially in cold climates.

Follow the manufacturer

The correct grade is in the owner's manual and often printed on the oil filler cap. Using a thicker oil than recommended does not give "extra protection" in a modern engine — it can slow oil flow at startup and reduce fuel economy.

What shops and stores should stock

Keep plenty of 0W-20, 5W-20 and 5W-30 for passenger cars, plus 15W-40 for diesel customers. Add 5W-40 synthetic if you serve diesel pickups or European vehicles.

Motor oil by the case

RPS Distributors carries motor oil from Castrol, Mobil 1, Pennzoil, Quaker State, CAM2, Shell, Gulf, Sunoco, Citgo, Motorcraft and Super S. Browse our Lubricants collection.

Request a wholesale quote or call 1-855-498-7509.