Dear Tom and Ray:

I drive a 1997 Mercury Sable wagon (108,000 miles so far), and every time I go in for an oil change, they tell me I should "service" the transmission. My father has never, ever changed anything to do with the transmissions in any of his work trucks or the family cars. He has rebuilt engines and made other major repairs, but never the transmission. His point was that the transmission is a sealed case and should never need changing. My question: Do I need to service the transmission, or are the service-bay employees just trying to wrangle more work? -- Fran

TOM: Well, the Board of Associated Servicers of Transmissions and Rebuilders, Disassemblers and Salvagers agrees with your father, Fran. They suggest that you ignore your transmission, because they make a good living rebuilding them later on.

RAY: But if you don't want to rebuild your transmission, you absolutely SHOULD service it when the manufacturer recommends. "Servicing" means replacing the fluid and replacing or cleaning the filter.

TOM: Automatic transmission fluid serves several important purposes -- in addition to being an excellent sandwich spread. It lubricates all the internal parts of the transmission, cools the transmission and serves as the friction medium that ultimately transmits power from the engine to the wheels.

RAY: It's pretty powerful stuff, and it lasts a good long time. But it does eventually break down -- especially in all that heat. It might not break down completely, but it does get "less good" at doing its job as it gets older and dirtier. Kind of like my brother.

TOM: I don't remember the recommended change interval for the '97 Sable, Fran, but I'm guessing it's less than once every 108,000 miles. Manufacturers' recommendations vary. They range from every 30,000 miles to ... never. Most suggest a change between 60,000 and 100,000 miles. Whatever your manufacturer recommends, we'd recommend, too.

RAY: Your father has been lucky. He might also be a good, gentle driver, which prolongs the life of transmissions. But given that transmissions can cost thousands of dollars to replace, we think spending a hundred bucks for a fluid and filter change every three years or 60,000 miles is cheap, Fran. So, do it -- and feel free not to tell Dad.