AMSOIL Signature Series 5W-40 Max-Duty Diesel Oil
100% synthetic diesel oil option with strong cold-start flow. Verify the required viscosity and specification for your exact engine before ordering.
View 5W-40 at AMSOILDiesel Oil Comparison
5W-40 vs. 15W-40 Diesel Oil
Compare two popular diesel viscosities without losing sight of the engine manufacturer’s required specification.
Featured AMSOIL products
These products are relevant to this guide. Click a product to view current specifications and availability for the selected application.
AMSOIL Signature Series 15W-40 Max-Duty Diesel Oil
100% synthetic diesel oil option for severe-duty applications. Confirm your engine's required viscosity and specifications before ordering.
View 15W-40 at AMSOILProduct images courtesy of AMSOIL INC. Product availability and specifications can change; verify fitment before ordering.
5W-40 vs. 15W-40 at a glance
5W-40
Flows more readily during cold starts while providing SAE 40 viscosity at operating temperature.
15W-40
A traditional heavy-duty diesel viscosity that may be appropriate in warmer climates or where specifically required.
The specification matters more than the label comparison
Choose from the viscosities approved by the engine manufacturer, then verify that the oil meets the required performance specification. Newer emissions-equipped diesels can have specific oil chemistry requirements.
Related AMSOIL guides
Frequently asked questions
Which is thicker, 5W-40 or 15W-40?
At operating temperature both are SAE 40 oils. At cold temperatures, 5W-40 flows more readily than 15W-40.
Can I switch from 15W-40 to 5W-40?
Only when the engine manufacturer allows that viscosity and the oil meets the required performance specification. Check the owner’s manual and application lookup.
Which is better for cold weather?
When both viscosities are approved for the engine, 5W-40 generally offers better low-temperature startup flow.