0W-40 is an SAE engine-oil viscosity grade that flows like a 0-weight oil in cold weather and acts like a 40-weight oil at operating temperature.
Engine oil labels look like a code, but 0W-40 oil decodes into two separate promises. The “0W” describes performance in cold conditions, and the “40” describes the oil’s thickness when the engine is hot. SAE, the Society of Automotive Engineers, sets the measurement bands that both numbers must meet.
What Do the “0W” and “40” in 0W-40 Mean?
The “0W” is a cold-weather rating, and the W stands for winter, not weight. It is a 0-grade cold-flow oil, not a “0-weight” oil.
The “40” is the high-temperature grade. That thicker hot film is why many European and performance engines call for a 40-grade oil in the first place.
What Is 0W-40 Used For?
0W-40 is used when a vehicle maker wants easier cold starts plus a thicker 40-grade film at operating temperature. The oil flows in winter like a light 0-grade oil, then thickens up enough to protect at high heat and load.
Suitability is decided by the vehicle manufacturer’s required viscosity and approvals, never by the grade alone. Product pages for oils like Red Line’s 0W-40 list specific applications such as VW/Audi, BMW LL-01, Porsche A40, Mercedes-Benz 229.5/229.3, and Chrysler MS-10725. If your owner’s manual lists 0W-40 with one of those approvals, the oil is a match; if the manual says something else, follow the manual.
Real 0W-40 Specs From Current Products
The same 0W-40 grade contains different finished oils. Viscosity, cold-flow, and volatility vary noticeably between brands, as the table below shows.
| Finished 0W-40 Oil | Viscosity At 100°C | Cold & Other Specs |
|---|---|---|
| RAVENOL Super Synthetik Öl SSL | 13.25 mm²/s | |
| Mobil 1 ESP Formula 0W-40 | SAE 40 band certified | Mobil identifies it as SAE 0W-40; full specs in Mobil’s U.S. product pages |
| Red Line 0W40 Motor Oil | 15.7 cSt |
Two things are worth noticing. First, Red Line sits near the top of the 40 band at 15.7 cSt while RAVENOL stays near the bottom at 13.25 cSt — both are legal 0W-40 oils, but they behave differently under load. Second, a strong cold-weather number doesn’t predict high-temperature behavior; the two sides of the grade are tested separately.
The differences matter when you pick a product. If you’re ready to buy, our tested roundup of the best 0W-40 synthetic oil options compares the top brands side by side.
Choosing An Oil: What To Check First
Three common mistakes cause most 0W-40 confusion: treating 0W as “0 weight,” assuming every 0W-40 is interchangeable, and confusing API category with viscosity grade. All three are wrong.
The additive package and OEM approvals differ from one 0W-40 to the next, so being the right viscosity is only half the job. API also keeps oil categories separate from viscosity grades — the current gasoline category is API SP, introduced in May 2020, and a 0W-40 oil can carry any current or older category. Never infer diesel suitability, emissions-system compatibility, or fuel economy from the grade alone; those depend on the exact specification and the manufacturer’s own recommendation.
The rule in one line: use 0W-40 only if the engine manual says so, and then pick a product that carries both the required viscosity and the required API and OEM approvals.
References & Sources
- American Petroleum Institute. “Motor Oil Guide.” Explains viscosity grades, the W rating, and how oil categories are defined separately from viscosity.
FAQs
Can I use 0W-40 instead of 5W-40?
Only if the manufacturer permits it. Both oils share the same SAE 40 hot-temperature band, so the main difference is cold-flow, but the engine’s required OEM approvals matter more than the viscosity swap. Check the owner’s manual for the exact grade and approval codes before changing.
Does 0W mean the oil weighs nothing at freezing?
No. The W stands for winter, not weight, and the 0 reports how well the oil flows at low temperature. A 0W oil’s cranking and pumpability limits are tested at -35°C and -40°C. The number still describes a measurable resistance to flow, just a much smaller one than a 10W or 20W grade.
Is 0W-40 good for high-mileage engines?
Not automatically. Mileage matters less than what the engine was designed to use. A high-mileage engine that calls for 5W-30 should keep using 5W-30, with a high-mileage variant if desired. The 40-grade hot viscosity is only correct when the vehicle maker specified it.