Using the proper oil in your engine...

Dear Car Talk

Dear Car Talk | Jan 01, 1990

Dear Tom and Ray:

I took my 1989 Mercury Sable for its first six-month service at 2,800 miles. The warranty booklet says to use 5W-30 oil in the engine. The dealer used 15W-40. When I questioned it, they said they use 15W-40 in every car. I had the service manager look it up and he agreed that 5W-30 was specified, but said 15W-40 was OK. Winter temperatures here are 20-40 F. Is it OK? Should I go to a different dealer next time?

RAY: No, it's not OK. And if you can't get this dealer to use the recommended oil, you should go to someone else. The two numbers refer to the viscosity--or how easily the oil flows. Oils used to be made only in single weights. So you would have to use a thin oil (like 5 weight) in the winter, and then drain it out and put in a heavy oil (like 30 or 40 weight) in the summer.

TOM: Now through the magic of chemistry, oils come in multi-viscosities. So a 5W-30 oil, for example, acts like a 5 weight oil when it's cold and like a 30 weight oil when it's hot.

RAY: Some car makers--including Ford and General Motors--now believe that 40 weight oil is simply too thick to give adequate lubrication to the internal engine parts of their cars. They've also found that lighter oils produce better fuel economy, and reduce pinging associated with the additives in 40 weight oil.

TOM: 5W-30 is recommended for your car, and that's what you should use. I'd trust the people in Detroit who designed the car over a dealer who may have gotten a closeout price on 4000 cases of 15W-40.

Get the Car Talk Newsletter