Expert Advice · 5 min read
5W-30 vs 10W-40 — which engine oil does your car actually need?
Those numbers on the bottle aren't marketing — they describe exactly how the oil behaves in your engine, from a frosty Canberra morning to a 40° summer run up the Hume. Here's what they mean and how to choose.
What the numbers mean
An oil like 5W-30 is a multigrade: the first number (5W — the W is for winter) tells you how easily it flows when cold, the second (30) how thick it stays at operating temperature. Lower first number = faster flow at cold start, which is when most engine wear happens. Higher second number = a thicker protective film once the engine is hot.
So a 5W-30 flows quickly on a cold morning and stays moderately light when hot — great for fuel economy in modern engines. A 10W-40 is a little slower to circulate when cold but holds a thicker film at temperature — often better for older or higher-kilometre engines.
Quick comparison
| 5W-30 | 10W-40 | |
|---|---|---|
| Cold start flow | Excellent | Good |
| Hot film thickness | Moderate (economy-focused) | Thicker (protection-focused) |
| Typical use | Most modern petrol & diesel cars | Older engines, higher kilometres |
| Fuel economy | Better | Slightly lower |
Viscosity is only half the story
Two oils with the same viscosity can be very different products. The other half is the standard your manufacturer specifies:
- ACEA C1 / C2 / C3 — low and mid SAPS oils for European engines and anything with a diesel particulate filter (DPF). Using a non-DPF oil in a DPF car can clog the filter — an expensive mistake. Our Enviro+ C1, C2 and C3 cover these.
- API SP / ILSAC GF-6 — the current standard for modern petrol engines, covered by Enviro+ GF-S 5W-30 and Enviro+ 0W-20.
- Heavy-duty diesel — older utes and 4WDs without a DPF often do best on a robust 15W-50 or even 20W-60.
So which one do I buy?
- Check your owner's handbook first. It lists the viscosity and the standard. Both must match.
- Modern car (roughly 2010 onwards)? You'll almost certainly be in 0W-20 or 5W-30 territory with an API SP or ACEA C-series requirement.
- Older or high-kilometre engine? A thicker oil like 10W-40, 15W-50 or 20W-60 helps maintain oil pressure and reduce consumption.
- Still unsure? Use our oil finder — pick your make and model and we'll match a genuine Penrite oil to it.
General guidance only — always confirm against your vehicle manufacturer's specifications.