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Copied from the web (MSN) (BTW: Their change intervals are for domestics - I would go with every 60-70K miles)

Trouble-Free Transmissions
Just as oil and filter changes are good for the engine, regular fluid and filter changes are necessary for the transmission.
by Peter D. duPre
If you've ever had an automatic transmission go out on you, then you are familiar with expensive repairs. Rebuilding a broken automatic transmission is costly, with repairs generally costing between $1,500 and $2,000—or more! But why pay this kind of money if you don't have to? The transmission in your vehicle is designed to last the life of your car or truck … if you take care of it.
Former Indy car mechanic Dave Bowman says that simply by checking the transmission fluid on a monthly basis and getting regular transmission fluid and filter changes, you can keep your transmission trouble-free. He advises car owners to check their owner's manual to see what the service interval is on their vehicle's transmission and to follow it.

If you own an older vehicle or don't have an owner's manual, Bowman suggests checking with your mechanic about service intervals or following the two-year rule.

"Ideally, the fluid and filter in the transmission should be changed every two years or 24,000 miles," says Bowman, "particularly on older vehicles or on those driven under harsh conditions."

"There are many different transmissions," says Bowman, "and even those offered by the same manufacturer may have different recommended service intervals. And some newer vehicles have transmissions that are designed to be service-free for the live of the vehicle. That's why it is important to check the owner's manual."

A fluid approach
If your vehicle is driven hard, tows a trailer, carries a camper or goes off pavement, however, by-the-book service may not be enough. Under these conditions it may be necessary to change the fluid and filter every 12 months or 12,000 miles as dirt and moisture buildup in the fluid can cause internal damage. Heat buildup can also be a problem. The harder the transmission works (such as when towing or hauling a camper), the hotter the fluid gets. If it gets too hot, it scorches and can seriously damage the transmission.

According to Ron Sessions, author of the "Turbo Hydra-Matic 350 Handbook" (HP Books), the ideal operating temperature for transmission fluid is between 175 and 225 degrees Fahrenheit.

"Anything other than light-duty use raises the fluid temperature beyond this," says Sessions. "Next to regular fluid and filter changes, installation of an automatic transmission fluid cooler can go a long way towards increasing transmission life, particularly if you tow a trailer."

Like Bowman, Sessions says that regular fluids checks are vital.

"Checking transmission fluid is a bit different from checking engine oil even though both are checked by a dipstick," said Sessions. "Engine oil is checked with the engine off, but transmission fluid is checked with the brake set, gear selector in Park, and engine running."

For an accurate check, the fluid should be at operating temperature and the vehicle should be parked on a level surface. Drive for about 15 minutes beforehand to bring the fluid up to temperature before the check. Pull the dipstick out and wipe it clean, then reinsert it. The level should read between the ADD and FULL marks. If it isn't up to level, add the recommended fluid via the dipstick tube. To do the job yourself you'll need a specially designed funnel (available at auto parts stores).

Don't add just any transmission fluid. Different vehicles use different types of transmission fluid, such as Dexron III or Mercon. The owner's manual will tell you which type of fluid to use. Pour it slowly into the funnel, checking often to make sure you don't add too much. Check the fluid at least once a month, topping off as necessary. If you find you are adding fluid on a regular basis, you may have a leak and should get the unit serviced.

Driving habits a factor
Checks and changes of fluid are an important part of transmission care, but so are driving habits. According to Bowman, one common practice that really hurts the transmission is shifting from Drive to Reverse while the vehicle is still moving.

"This is really hard on the transmission," says Bowman. "Always make sure your foot is on the brake and the vehicle is stopped before shifting into reverse."

Manual downshifting into Low can also be bad. Many vehicles aren't designed for low-range driving except under certain conditions, such as towing. Check your owner's manual.

Another practice that shortens transmission life is parking without using the parking brake. This is an especially bad when the car is parked on a slope as all the vehicle's weight is on a tiny metal catch inside the transmission. If the catch snaps under the strain, the vehicle can roll downhill and hit something.
 

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I read this article last week and my conclusion was that Mr. duPre needs

some lessons in theoretical operation and real life use. The transmission is a very complex system and in theory it will last forever. Afterall, unlike the crankcase, it's a sealed system and there is no reason to believe that the fluid will be contaminated. However, in the real world, we tow, accelerate hard and push these mechanisms. Even persistent regular maintenance will not save an autotranny. My advice, tranny service at least every 70-100K miles.
 

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I totally agree with changing the transmission fluid. I just had the dealer change my transmission fluid, and differential F/R for $400. I also Have a 1994 Honda accord that I change the tranny fluid every year.. I now have 160K miles original tranny, original engine (synthetic oil) car runs like it is two year old.
 

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Transmission oil becomes contaminated, over time, due to gradual wear of the 'brake bands'...these are components that are lined with a friction material in the tran's that operate every time the 'box changes gear (as an analogy, they work in a similar fashion to brake pads on the rotors on the braking system)

Hence, when i changed my trans' oil there was a certain amount of 'sludge' in the sump...
 
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