Can you use 15w40 oil instead of 20w40 in diesel engine?

Yes, you can use 15W-40 oil instead of 20W-40 in a diesel engine, as both oils have similar viscosity characteristics at operating temperatures. However, the lower viscosity at cold temperatures in 15W-40 may provide better cold start performance in cooler climates. Always check the engine manufacturer’s recommendations to ensure compatibility and optimal performance.