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Dear Tom and Ray - As a faithful practitioner of the 3,000-mile-oil-and-filter-change rule, I believed that my reliable 1988 Toyota Corolla and I would be together for many more years to come. Now, to my chagrin, I have been told by a friend that this may not be a certainty. My situation is this: I drive the car sparingly (approximately two miles a day), so my last oil change was in October of 1994. I am writing to you in April of 1995, and I still have 1,000 miles to go before my next oil change. Now, however, I've been told that all of this short-distance driving can be more damaging to my car's oil than harder, longer drives. Is this true? And if so, what should I do? - Mark

TOM: It is true, Mark. Driving short distances all the time IS harder on your engine.

RAY: When you start the car in the morning and drive only a mile or two, several things happen. First, the engine runs rich when it's cold, so there's extra gasoline pouring into the cylinders. And not all of that gas gets burned, so some of it sneaks past the rings and ends up mixing with the oil.

TOM: Second, water is a by-product of combustion. So you also get water sneaking down past the rings and diluting the oil.

RAY: Normally, that's not a problem, because when the engine reaches operating temperature, the gasoline and water are vaporized and purged through the crankcase ventilation system. But here's the rub: When you only drive the car a mile or two, the engine never reaches operating temperature, so that excess gasoline and water never get out. Instead, they stay in the crankcase and dilute the oil.

TOM: That's why we, and most manufacturers, recommend a mileage- or a time-based oil change interval. We recommend 5,000 miles or six months, whichever comes first (we used to recommend three months or 3,000 miles, but oils have gotten so much better, and disposal of old oil has gotten so much more difficult, that we've updated our numbers).

RAY: So in your case, if you change the oil and filter every six months, your Corolla should be fine. In fact, you'll probably be driving it well into the second Clinton administration - Chelsea Clinton's second administration, that is.

Dear Tom and Ray - I have a question about the difference between "twins." You know, like the Ford Taurus/Mercury Sable twins and the Plymouth Voyager/Dodge Caravan duet. Are these twins exactly the same? If so, why the price difference? - Tom

RAY: They ARE basically the same, Tom. The price difference is mostly a marketing tactic. For example, Mercury is positioned as more "upscale" than run-of-the-mill Ford.

TOM: The way they make the Sable more upscale is to include a bunch of options in the base model that the Taurus does not include in its base model. The most significant is air conditioning. But when you put the same options in both cars and add up the costs, the Sable ends up costing about $400 more.

RAY: And for that $400, you get slightly different styling, you get the name "Mercury" on your car, and you get a tachometer.

TOM: Of course, your neighbors will also think you're very successful when they see you pull into the driveway in a Mercury, as opposed to a plain, old Ford.

TOM: Hey, do you think you're taking good care of your car?

RAY: If you're like many of our customers, you may be ruining your car without even knowing it. Yes, even you! Find out how. Send for your copy of our informative pamphlet, "Ten Ways You May Be Ruining Your Car Without Even Knowing It!"

TOM: Send $3 and a stamped (52 cents), self-addressed No.10 envelope to Ruin No.1, PO Box 6420, Riverton, NJ 08077-6420.