This is what I believe when it comes to changing transmission. These are my firmly held beliefs and pretty much nothing is going to change them. I posted it about a year ago on another forum, so I'm copying and pasting it here:
The myth is old fluid has clutch material in it, and putting fresh fluid in it leaves not enough clutch material thus it starts slipping.
I have a few problems with this myth: First of all, this myth started like this: Some guy never maintains his transmission properly and never changes the fluid (BIG no-no ) and eventually he starts having problems. At this point his trans is toast from poor maintenance and nothing will save it. But he thinks “Oh maybe I should change the fluid and it’ll fix it.” So he gets the fluid changed, his trans dies shortly after so he thinks it must be from the fluid, not the years of neglect.
Second of all, the ONLY place you want clutch material is on the clutches. Period. You do not want it floating around in your fluid. Over time, clutch material WILL wear off the clutches and will eventually get bad enough to make the trans slip. This is INEVITABLE and is the reason NO automatic can possibly live forever. But one thing is for certain: You DO NOT want clutch material floating around in your fluid. So change your fluid frequently and keep it fresh. If you change fluid on an old transmission and it begins slipping immediately, your trans was already toast in the first place.
If your trans is still working perfectly fine, changing fluid WILL NOT HURT IT!! Like I said, if your trans ides shortly after, it was already dead to begin with. Also, just because the trans is working fine, do not think that you don’t have to ever change the fluid. Change transmission fluid every 30-40k miles, whether the fluid “looks” good or not. You wouldn’t wait until your motor oil turns to sludge before thinking about changing it would you?
And FINALLY: Transmission flushes are NOT a bad thing. The term “flush” is pretty misleading. Flush machines don’t actually “flush” your trans per se, all they do is exchange the fluid. Here’s how it works: Your trans has a pump and a cooler. When your driving, the trans pumps fluid through the cooler. When you get a flush, they hook up the cooler lines to the flush machine. Then they start the engine. Once again, the trans uses it own pump to pump the fluid through the cooler lines, just as it normally does. But, all of the fluid getting pumped out gets dumped into a tank on the machine. All the machine does is make sure the fresh fluid gets pumped back into the trans at the same rate it’s coming out. There isn’t any actual “flushing.” It’s a perfectly safe method and is really the only way to change all the fluid in the trans at once.
