Dear Tom and Ray:
I have a 1989 Jaguar XJ6. And every 3,000 miles when I change the oil, I also drain out 3 quarts of the automatic-transmission fluid by removing the transmission drain plug. Then I put in 3 new quarts of ATF to replace it. I'm hoping this will prolong the life of the transmission. Am I correct? -- Eliot
TOM: Well, you're certainly prolonging the life of the company that makes the automatic-transmission fluid, Eliot.
RAY: And you're also lengthening the life of your transmission. As you drive the car, the transmission fluid slowly gets contaminated. That's normal. But as the miles pile up, it gets more contaminated, and does a lousier job of lubricating.
TOM: What you're doing is draining out some of that fluid every 3,000 miles. You can't get all of it out because of the way the transmission is configured -- your car's transmission holds 8.5 quarts -- but you're draining out about a third of that contaminated ATF, and refreshing the mixture with some new stuff.
RAY: It's easy to do, and it probably does have some benefits. In fact, I think I smell a new, $39.95 recommended service at our garage!
TOM: An even better case can be made for doing it on your car, Eliot, since a rebuilt transmission for this beast would probably cost you as much as a one-bedroom condo in a midsize city.
RAY: Now, if you could only change a third of the electrical system every time you change the oil ...