Dear Tom and Ray:

I am looking to buy a car soon, and I am thinking about getting a diesel, because of their great gas mileage. I know a thing or two about cars, but I'm not a professional mechanic yet, as I'm still in high school. I've heard that it's much more expensive to change the oil in a diesel than a regular gasoline engine. Is that true? Thanks. -- Phil

RAY: In general, yes. Diesel engines are harder on oil than gasoline engines. They run dirtier, and the oil gets filthy faster because soot blows by the piston rings and into the crankcase.

TOM: So, most manufacturers of diesel engines recommend that you change a diesel's oil more frequently than you need to change the oil in a gasoline engine.

RAY: Secondly, there's more oil to change on a lot of diesels. Most diesel engines hold one and a half to two times as much oil as their gasoline counterparts. So, for instance, if you're changing the oil in your Dodge Ram with a gasoline engine, you'll need seven quarts of oil. If you have the Cummins diesel engine, you'll need 12 quarts.

TOM: Finally, there's the type of oil the engine uses. Lately, more diesel-engine manufacturers have been switching over to synthetic oil, because it lubricates better and can go longer between changes. That's why the Jeep Liberty Diesel's oil needs to be changed only every 12,500 miles. An improvement, right?

RAY: Sure. But here's the rub: Synthetic oil costs four times as much as regular, old, dinosaur oil.

TOM: So, while you might have to change the oil on your diesel Liberty only every 12,500 miles, it's going to cost you 80 or 90 bucks to have someone drain your crankcase and refill it with 6.3 quarts of synthetic oil.

RAY: The specifics vary from manufacturer to manufacturer, and even from car to car. For instance, the VW Jetta recommends synthetic oil for both the diesel AND gasoline engines it offers. And the oil-change intervals for the two types of engines are identical. So we can't give you an absolute answer that will be true for every diesel engine.

TOM: But you need to do the math and ask some questions before you go diesel. Ask which type of oil is recommended, the oil capacity and the recommended oil-change interval. You'll probably get better mileage with a diesel, but in most cases, some of that savings will go toward covering higher oil-change bills.