5W-40 vs. 15W-40 Diesel Oil: Which Should You Use?
Understand the difference, then match the oil to your exact engine.
Check your exact model year and required specifications. View current prices and order directly at AMSOIL.
Quick answer: 5W-40 vs. 15W-40
Choose from the oil grades approved for your exact engine and model year. When both are permitted, 5W-40 meets more demanding winter-grade requirements, while both oils carry an SAE 40 operating-temperature grade. Towing alone does not determine the correct grade.
When your engine allows both grades, 5W-40 meets lower-temperature winter-grade requirements than 15W-40. Both meet the SAE 40 grade at operating temperature. That does not mean their measured viscosity or other properties are identical.
Start with the engine manufacturer's requirements. Climate matters within the approved choices; it does not override them.
5W-40 vs. 15W-40 diesel oil comparison
| Question | 5W-40 | 15W-40 |
|---|---|---|
| Winter grade | Lower-temperature winter-grade requirements. | Higher winter grade; check the approved temperature range. |
| Operating-temperature grade | SAE 40. | SAE 40. |
| Cold-weather use | Worth considering when approved for your engine. | Use only within the manufacturer-approved conditions. |
| Towing or heavy service | Must meet the required specification and service guidance. | Must meet the required specification and service guidance. |
| Correct choice | Depends on year, engine, specification and conditions. | Depends on year, engine, specification and conditions. |
How to choose diesel oil for Cummins, Power Stroke or Duramax
- Find the approved viscosities and temperature guidance in your owner's manual.
- Check the required oil specification, including any engine-specific requirements.
- Confirm the exact year, make, model and engine in the product lookup.
Do not apply one Cummins, Power Stroke or Duramax recommendation to every model year. If the manual and lookup disagree, resolve the difference with the manufacturer or AMSOIL before ordering.
Compare the AMSOIL products
After confirming the approved grade, check the current specifications, package sizes and price for the product you need.
Do not choose from the grade alone
Two oils with the same grade can carry different specifications. Emissions equipment and engine design can affect the required product. A higher winter-grade number is not proof of better hot-weather or towing protection.
Reference: AMSOIL oil and viscosity FAQs. Your owner's manual supplies the requirements for your engine.
Common questions
Which is thicker, 5W-40 or 15W-40?
Both meet the SAE 40 operating-temperature viscosity grade. The 5W winter grade meets lower-temperature requirements; the oils are not necessarily identical at every temperature.
Can I switch from 15W-40 to 5W-40?
Only if your engine manufacturer allows that viscosity and the product meets the required specifications for your exact application.
Is 15W-40 always better for towing?
No. Towing alone does not determine the viscosity. Use the engine manufacturer’s viscosity, specification and severe-service guidance.