Every oil container has a code like “5W-30” or “0W-20” — but most drivers don’t know what these numbers actually mean or why they matter. Understanding oil viscosity takes five minutes to learn and can help you avoid one of the most common oil change mistakes: using the wrong grade for your vehicle.
What the Numbers Mean
The “W” stands for “Winter” — not “weight” as many people assume. The number before the W indicates how the oil flows in cold temperatures. The lower the number, the better the cold-flow performance:
- 0W — Flows very freely at cold temperatures; best for cold-climate cold starts; most modern European vehicles
- 5W — Standard cold-weather performance; most common in modern North American vehicles
- 10W — Slightly thicker at cold temps; common in older vehicles and some diesel applications
The number after the W indicates viscosity at operating temperature (100°C). Higher number = thicker at operating temperature:
- -20 — Very thin at operating temperature; common in modern fuel-economy-optimized engines; requires full synthetic to maintain film strength at this low viscosity
- -30 — Standard for most modern passenger cars
- -40 — Slightly thicker; common for high-performance engines, European vehicles, and some diesel applications
- -50, -60 — Heavy-duty and performance applications; not typical for passenger vehicles
Why the Right Viscosity Matters
Oil that is too thin at operating temperature may not maintain adequate film thickness on high-load bearing surfaces. Oil that is too thick flows slowly on cold starts, delaying pressure buildup to critical components. Either condition accelerates wear. Modern engines are engineered around specific oil viscosities — using the wrong grade isn’t just wasteful, it affects protection and can affect fuel economy (too thick) or longevity (too thin).
Always use the viscosity specified in your owner’s manual. Norm’s Auto Clinic — 19 N. Broadway, Coweta, OK 74429. Call (918) 279-8100. We look up your vehicle’s specification and use the correct oil every time.