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Originally Posted by RevJunky
That is what the dealer told me and I think it's complete nonsense. That might have some truth in it if you use the wrong fluid but I doubt changing your using the proper fluid will create any new problems that weren't there to begin with. If there is a major problem, changing your fluid probably won't cure it, it might just delay the inevitable. There is no way to determine if changing the fluid will make the transmission last longer but it's certainly worth the peace of mind that it will.
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As transmissions wear, the clutch pack and band material residue collects in the transmission. Most of it is in the pan, where it sits and is completely harmless. Some of it can be in the valve body, which has many small orfices and valves and controls the transmission operation. It looks like a maze, with check valves, etc. The problem with draining a transmission is that it isn't like an engine, it doesn't all drain down and come out the bottom. You will get something like 40% of it out by draining it, and the rest is in the torque converter, cooler, lines, valve body, etc. To change all of the fluid, you either need a pressure changer, or you need to do multiple manual fluid changes, increasing the % of new fluid each time. When you put in new fluid, and start it back up, you can often dislodge the sediment that is sitting in the valve body. This results in a subsequent transmission failure. It is a well-understood cause of transmission failure.
Theoretically, cleaner fluid is always better than dirty fluid. No one is arguing that. Let's assume you put the correct fluid in, and take that issue off the table. The problem is that you have to go through the process of changing the fluid to get the clean fluid in there, and it is the changing process that causes failures, particularly with high mileage transmissions (which have more sediment in them).
Some believe that the benefits outweigh the risks, and go ahead and change the fluid anyway. Some just aren't aware of the risks. BMW (and some of us) believer that the risks are greater than the benefits, and suggest it be left alone. It is not a cost of maintenance issue, for BMW or whoever is paying, it is a risk issue. I don't believe that dirty fluid is more likely to cause a transmission failure than clean fluid, within reason. I believe that the transmission will probably fail due to electrical sensors, valve body problems, actuators, and wear out, more than due to bad fluid. Cleaner fluid won't prolong the life of any of those components.
There is a history of transmission failures at something greater than 100k miles. I have seen no data that suggests that those failures are due to dirty fluid. I have seen transmission failures soon after the fluid has been changed. Search the board for stories about posters with that experience, who put it down to bad luck that they had a transmission failure "even after they had just changed the fluid"
I do believe in preventative maintenance. I do change fluids where I see a benefit to doing so. I just don't see the logic in changing the fluid in a sealed unit, when the very act of changing it introduces a new failure mode that wouldn't exist if it was left alone. If it gives you peace of mind then it is worth doing it. I would have more peace of mind by not doing it myself. Just my $0.02.