0W-20 vs 5W-30 vs 10W-40: Oil Viscosity Explained
The numbers on the oil bottle are a code worth reading. A Cranbourne West specialist decodes it.
What the numbers mean
The first number with the W (“winter”) is cold-flow: lower flows faster on cold start, when most engine wear happens. The second is thickness at operating temperature: the protection film at running heat. So 0W-20 starts easier and thinner; 10W-40 runs thicker when hot.
Modern engines are built with tight clearances designed for thin oils — 0W-20 in a new Toyota isn’t a cost-cut, it’s the spec.
Why you can’t just go thicker
The old “thicker protects better” instinct damages modern engines: thick oil reaches tight galleries and VVT systems too slowly, starving exactly what it meant to protect. Equally, thin oil in an engine designed for 10W-40 won’t hold film. The viscosity your maker specifies — alongside the approval — is the answer, not folklore.
High-km engines occasionally justify one sensible step — a judgement call we make per engine, not a default.
Not sure which is right for your car?
Every car and situation is different — the best choice depends on your exact vehicle, how you use it and its condition. We give you straight, no-pressure advice based on what’s actually best for you, not what makes us the most.
Talk it through with a Cranbourne West specialist. Book online in 60 seconds or call 03 8782 0711.
0W-20 vs 5W-30 vs 10W-40: Oil Viscosity Explained — FAQ
Cold-flow before the W (lower = faster cold starts), hot thickness after it — the protection film at running temperature.
Not in modern engines — thick oil starves tight galleries and VVT systems on start-up. The specified viscosity is the answer.
They’re built with tight clearances designed for it — it’s the engineering spec, not a cost-cut.
Occasionally one sensible step on a worn high-km engine — a per-engine judgement, not folklore.
Related guides & services
More guides like this
Want honest advice?
Book your car in online in 60 seconds — RACV-accredited, fixed written pricing, no surprises. Or call 03 8782 0711.