5W-30 vs 5W-40 vs 10W-40
Use the grade the engine handbook specifies. If you are choosing among these three: 5W-30 is the thinner hot oil for modern petrol and many light diesels; 5W-40 keeps the same cold start as 5W-30 but a thicker film when hot; 10W-40 is thicker in the cold than either 5W grade and is common in older or hotter-climate engines. The numbers are SAE J300 limits, not marketing.
What actually differs
Every grade is two promises in SAE J300: a winter (W) rating for cold cranking and pumping, and a hot rating for viscosity at 100 °C plus a high-shear HTHS floor.
| Grade | Cold (W) | Hot KV100 | Typical use |
|---|---|---|---|
| 5W-30 | 5W — cranks/pumps at the 5W temperatures | 9.3–12.5 mm²/s, HTHS ≥ 2.9 mPa·s | Modern petrol, many light diesels, fuel-economy maps |
| 5W-40 | Same 5W cold window | 12.5–16.3 mm²/s, HTHS often ≥ 3.5 | Hotter, loaded, or turbo petrol/diesel where the OEM allows 40 |
| 10W-40 | 10W — thicker at start than 5W | Same 40 hot band as 5W-40 | Older engines, hot climates, some motorcycles (check JASO) |
The spec on the bottle (API SP, CK-4, ACEA C3, and so on) is a different decision from the SAE grade. You need both to match.
When each grade makes sense
Stay on 5W-30
Handbook says 5W-30 or 0W-20/5W-30. Tight clearances, VVT, DPF-equipped engines, and fuel-economy calibrations are built around the thinner hot film.
5W-40 if the book allows it
Same cold start as 5W-30, stronger hot film. Useful for towing, hot summers, or engines that list 5W-30 / 5W-40. Do not put it in an engine that only lists 5W-30.
10W-40 when cold flow is not the issue
Fine where winters are mild and the OEM lists 10W-40 or 15W-40. A poor substitute for 5W-30 in a modern petrol car that must oil the cam phasers quickly on a cold start.
Never mix the spec
A 5W-40 that is only API SN is not a substitute for a 5W-30 ACEA C3 even if the viscosity looks “stronger”. Aftertreatment chemistry matters more than the second number.
What people get wrong
- “40 is always safer.” Extra hot viscosity raises friction and can slow oil through fine galleries. Wear can go up, not down, if the engine was mapped for 30.
- “5W and 10W feel the same in India.” Many days yes. A 10 °C winter morning in the north, or a hill start, is where the 5W rating shows.
- Motorcycle 10W-40 often also needs JASO MA/MA2 because the oil wets a wet clutch. Car 10W-40 is not automatically clutch-safe.
For the chemistry behind the numbers, read engine oil technology and synthetic vs mineral.
5W-30 / 5W-40 / 10W-40 FAQ
Can I switch from 5W-30 to 5W-40?
Only if the manufacturer lists 5W-40 (or a 30/40 range) for that engine and climate. The cold behaviour is similar; the hot film is thicker. If the book is 5W-30 only, stay there.
Is 10W-40 better than 5W-30 in summer?
Not automatically. Summer heat is the hot number (30 vs 40), not the W number. A 5W-40 covers hot running without giving up cold starts. 10W-40 is the choice when the OEM lists it and cold cranking is not critical.
Do I need fully synthetic for 5W-30?
Most 5W-30 oils that meet modern API/ACEA specs use Group III or better. The spec and HTHS matter more than the word “synthetic” on the label.
Lubechem formulates 5W-30, 5W-40 and 10W-40 for manufacturers
If you want to see how a grade is built to SAE J300 and API/ACEA — not which bottle to buy — start with the engine-oil technology guide.
See how an engine oil is built →