Ask a Mechanic: Oil changes and diesel vehicles

Dear Ask a Mechanic,

I was looking at getting a diesel half-ton pickup. I’ve heard that it’s more expensive to get an oil change with a diesel vehicle. Why? What is different then with a gasoline engine? – Diesel doubter

You’re right that the cost of an oil change for a light-duty diesel pickup is typically higher than in a comparable gasoline model. At my workplace, the price varies, but it is fair to say that in some cases it can be twice the amount.

For example, changing the oil in one of GM’s latest inline six-cylinder models is currently more than $225, due in part to supply chain issues that force us to purchase oil directly from the dealership that meets the required specification of the automaker. In GM’s case, it is dexosD oil. This is just one of several factors that make oil changes more costly.

Diesel engines have unique needs when it comes to motor oil, due in part to the severe bearing loads and shear conditions (metal sliding against metal) that are created by their high combustion pressures and the need to pressurize their fuel systems to as much as 35,000 psi using engine-driven, mechanical pumps.

Additionally, the oil must be capable of suspending and holding the substantial amount of soot that’s generated by the large-scale use of exhaust-gas recirculation and it needs to do so in an engine that naturally takes longer to warm up, reducing contaminant burn-off time (and while still cooling and lubricating hot turbocharger bearings).

As a final challenge, the oil must produce minimal metallic ash when burned inside the engine and not contain compounds that would be detrimental to the exhaust emissions components that all modern diesels require, such as the particulate filter and selective catalytic reduction system. The challenges and costs involved in engineering and producing these synthetic oils and their additives all contribute to that bottom line. As well, most diesel engines require more oil than gas versions do.

If there’s a positive side, it’s that the Diesel may (emphasis on may) require less frequent servicing. Trucks made by the big three domestic automakers use a computerized oil life monitor instead of a mileage schedule, making it impossible for me to say for certain. But you can expect dramatically better real-world fuel consumption with a diesel, which should help offset the cost of the truck’s maintenance somewhat.

Ask a Mechanic is written by Brian Early, a Red Seal-certified automotive technician. You can send your questions to wheels@thestar.ca. These answers are for informational purposes only. Please consult a certified mechanic before having any work done to your vehicle.