Both 15W-40 and 10W-40 are multigrade diesel oils, and both reach the same thickness once the engine is hot. The difference is how they behave when the engine is cold.
What the numbers mean
The first number, before the W, shows how the oil flows at low temperature. A 10W oil flows faster than a 15W oil on a cold morning, so it reaches bearings and the turbocharger sooner. The second number, 40, is the viscosity at normal operating temperature. Here both grades are the same.
When 15W-40 is the right choice
- Hot and warm climates where cold starts are rare
- Older engines and high mileage fleets
- Heavy loads, long idling and dusty conditions
When 10W-40 makes sense
- Regions with cold winter mornings
- Engines that start and stop many times a day
- Fleets that want a small fuel economy gain
Always check the specification
The grade is only half of the decision. Make sure the oil meets the API, ACEA and OEM specifications in your vehicle manual. Extreme Diesel Oil 15W-40 meets API CI-4/SL and ACEA E7, which covers most trucks and buses in service today.
