Let me take a stab at this debate about not changing ATF on high mileage cars, which have never had it changed before.
Based on my research and endless reading, I finally think I got it. First, it is not because of the gunk in the tranny. As most cases have shown that the interior of the tranny is almost always clean, so there is virtually no gunk to dislodge.
Now here is the fun part. If you understand how the auto tranny works, you know that the friction of the gears is critical for smooth functioning of the tranny coupled with the correct ATF. That's also why ATF is more viscous than the manual tranny fluid and the correct ATF should be used for which the tranny is designed. Now imagine after a lot of miles, the gear friction would have changed from when new and the old ATF also has changed its viscosity from the new fluid.
Assume then you change ATF to new fluid but the gear friction is still matched to the old ATF viscosity. What would happen? Bamm, they don't work/match well any more.
Does this make sense? If this is truly the logic behind it, a partial fluid change on high mileage cars may work but it doesn't help in terms of how the tranny changes gears because they don't match well now. The semi new ATF, however, does provide benefits in case that the old ATF is completely burnt meaning losing all its functioning properties. Only under this situation, a fluid change may provide limited improvement. Remember that in this state there will be no good match between the worn gears and any fluid. Then, it is time for a new tranny.
Last edited by jzwu; 12-09-2011 at 10:18 PM.