0W-40 is thicker at operating temperature than 0W-30, protecting high-performance engines better, while 0W-30 boosts fuel economy in standard engines.
For the full breakdown, see our best 0W40 Engine Oil guide.
The difference between 0W-30 and 0W-40 motor oil comes down to one number: the viscosity at engine operating temperature. Both oils flow identically in freezing cold — that is what the “0W” means — but 0W-40 stays thicker when the engine is hot, forming a tougher film for turbocharged and high-horsepower engines. 0W-30 is thinner at operating temperature, reducing internal drag and improving fuel economy for naturally aspirated engines running moderate duty cycles.
0W-30 vs 0W-40 Viscosity: The Numbers That Matter
Both oils are fully synthetic and share the same cold-weather rating. The 0W rating means they pump effectively down to about -40°F (-40°C). The difference shows up at operating temperature. At 100°C (212°F), 0W-30 measures about 10–11 cSt, while 0W-40 runs closer to 14 cSt.
| Specification | 0W-30 | 0W-40 |
|---|---|---|
| Viscosity at 100°C | 9.3 – 12.5 cSt | 12.5 – 16.2 cSt |
| Typical 100°C value | ~10–11 cSt | ~14 cSt |
| Viscosity index | 160–170 | 170–185 |
| Ambient temp. range | -40°C to +30°C | -40°C to +40°C |
| Pour point | ~-45°C to -48°C | ~-42°C to -54°C |
| Flash point | >210°C – 220°C | >230°C |
| Energy to pump | Baseline | ~9% more |
The extra thickness in 0W-40 provides higher film strength under extreme heat but needs more energy to pump — which is why 0W-30 earns better fuel economy in everyday driving. The viscosity index (160–185 for both grades) keeps the oil stable across a wide temperature range.
Which Engines Need 0W-30 Versus 0W-40?
Standard naturally aspirated engines are designed around 0W-30. Vehicles like the Ford Focus, Nissan Qashqai, and typical 2.0L four-cylinders use the thinner oil to cut friction, improve cold starts, and hit their fuel economy targets.
Turbocharged and high-performance engines run hotter and demand a thicker film. BMW M models, Mercedes-AMG, Audi S3, and the 2.9L Alfa Romeo engine all specify 0W-40. The oil resists thermal breakdown at elevated temperatures and maintains protection under heavy load or track use. For owners whose engines call for 0W-40, our tested picks for the best 0W-40 engine oil cover top options for turbo and performance builds.
Climate also factors in. 0W-30 suits cold regions like the US Northeast and Canada. 0W-40 handles ambient temperatures up to about 40°C (104°F) and works well in the US Southwest or during summer towing seasons.
What Happens With the Wrong Viscosity?
Using 0W-30 in a turbo engine that requires 0W-40 risks the oil thinning out under extreme heat, reducing film strength and accelerating bearing wear. The reverse — running 0W-40 in an engine designed for 0W-30 — costs fuel economy and may slow cold-flow startup on bitter mornings, though it rarely causes immediate damage.
Older engines with wider internal tolerances sometimes benefit from 0W-40, which fills gaps better and cuts oil consumption. The owner’s manual is the final authority; straying from the specified grade can void a warranty.
FAQs
Can I mix 0W-30 and 0W-40 oil?
Mixing them creates a blend with viscosity somewhere between the two grades. It is not recommended as a long-term practice because it alters the oil’s engineered properties. In an emergency top-off, one quart mixed in is unlikely to cause harm, but the correct grade should replace it at the next change.
Is 0W-40 always better than 0W-30?
No. 0W-40 is better for engines that need it — turbocharged, high-performance, or heavy-load applications. In a standard naturally aspirated engine designed for 0W-30, the thicker oil reduces fuel economy and adds unnecessary drag without providing any protection benefit.
Does 0W-40 last longer between changes?
Some 0W-40 formulations have a higher total base number (TBN) and better thermal stability, which can support longer oil change intervals under normal driving. Actual intervals depend on the specific additive package and the manufacturer’s recommended schedule — always follow the oil life monitor or manual.
References & Sources
- carsTale. “0w30 Vs 0w40 Engine Oil: Which Performs Better?” Detailed comparison of viscosity differences and application suitability.
- AUTODOC (France). “Quelle difference huile 0w30 et 0w40” Technical specs including viscosity ranges, pour point, and flash point data.
- OilCapa City. “0W-30 vs 0W-40 Oil Comparison Tips” Real-world guidance on temperature thresholds and climate suitability.