Dear Tom and Ray:

The owner's manual for my 2004 Pontiac Vibe lists two maintenance schedules. The long-trip plan calls for the engine oil and filter to be changed every 7,500 miles (or 12 months), while the short-trip maintenance schedule sets a 3,000-mile interval for this service. Although my car meets all the requirements for the long-trip oil-change interval, the dealership where I take it strongly recommends the short-trip schedule in the winter, because according to the service manager, the winters in Maine cause oil to break down more quickly, and this can lead to engine damage if the oil is not changed every 3,000 miles. Is the dealership just looking for more business, or is it correct? -- Wayne

TOM: Yes, and yes. Of course it's looking for more business. But harsh winter conditions can be harder on your oil.

RAY: Most manufacturers now recommend a 7,500-mile oil-change interval for what they call "normal" service. That's the way most people use their cars -- including you, Wayne.

TOM: Then they have what's usually called an "extreme duty" or "hard service" interval of 3,000 miles for people who make their engines work the hardest. These are folks who use their cars as taxicabs, drive on dirt roads a lot or tow trailers frequently. Normally, spending a winter in Maine is not considered extreme duty. Not for the car, anyway. For me it would be.

RAY: But what tends to happen in the winter is that people drive places they usually don't drive. In the summer, you might walk 10 minutes to the store to pick up a quart of vegan peppermint patties. But when it's 2 degrees out with a 50 mph wind, and your thighs freeze together after your first 10 steps, you'd probably decide to take the car. And when the car is used for lots of short trips in cold weather, the engine never really warms up entirely.

TOM: Until an engine warms up and reaches full operating temperature, it doesn't burn its fuel efficiently. That means there's unburned gasoline in the cylinders. Some of that gasoline makes its way past the piston rings and into the crankcase, where it mixes with the oil. That mixture makes for a lousy lubricant.

RAY: So, it's not the winter temperatures themselves that damage the oil, it's the type of driving people tend to do in the winter.

TOM: So if you make a lot of short trips like that in the winter, then it might make sense to change the oil a little more frequently. We recommend 5,000-mile intervals to all but the most extreme drivers. It's a decent balance between protecting your engine and not overburdening the environment with used oil. That should be plenty often, even in the tundra where you live, Wayne.