Dear Tom and Ray:

I have a 2001 Lexus RX 300. I am out of the country most of the year, so I don't generate the miles necessary to require an oil change. However, my owner's manual suggests that the oil be changed every year anyway. Oil doesn't wear out if it's just sitting there, right? So is it really necessary to change the oil annually, even if you don't drive the required 5,000 miles? And if so, why? -- John

RAY: Well, the answer is yes, John. And the reason is water. Even though your oil doesn't break down from being used over many miles, it does get contaminated by water.

TOM: Every time you drive the car, water is created by the combustion process. And some of that water inevitably gets into the crankcase and mixes with the oil. Under normal circumstances, the next time you really heated up the engine, the water in the crankcase would evaporate and be purged by the crankcase ventilation system. But since you rarely drive your car, water can build up in your crankcase.

RAY: That's bad for a couple of reasons. No. 1, water is a lousy lubricant -- at least compared with oil. So by driving the car with a mixture of water and oil as your lubricant, you risk doing internal damage to your engine.

TOM: With a mixture of oil and water in there instead of just oil, you could put 10,000 miles' worth of wear on the engine during your 1,000 miles of annual driving.

RAY: The second problem with water is that it mixes with nitrates from the combustion process and forms acids. Those acids can attack metal parts of the engine and cause rusting and corrosion.

TOM: The bottom line is that light use of your engine (especially if it's used for short trips) can sometimes be harder on the oil than heavy use.

RAY: So here's what I'd do: Every year, as soon as you get back into the country, change the oil first thing. That way, you'll always be driving on good, clean oil.

TOM: Think about it this way, John. You bought a $35,000 car. You jet around the world all year. In order to save 30 bucks a year on an oil-and-filter change, you're putting a $6,000 engine at risk? Even Bill Bennett might walk away from that bet, John.