Overfilling can have effects such as heat as well. Most folks think that "more = better", so they tend to fill things to the top, or more. However, some trannies don't like to be "full".
Case in point where I have specific experience firsthand is the Allison 1000 series behind my Dmax ...
The "correct" method (per some folks I have had direct conversations with who work here in Indy at the Allison center) is to get the tranny up to around 160 degF, and then set the level to the bottom of the "hot" range on the dipstick. This allows for heat expansion to not "overflow" the mechanical operating group in the tranny. As the temps can move safely up to 225 degF or more, then the Allison will not overheat it's fluid and burp out via the vent.
And I proved this to be true. My tranny was running a bit warm (just a bit over 200F) during a long drive westward on I-90. At the campground that evening, after it had cooled to 160 degF, I drained out about two quarts to reset the level at the bottom of the temp range on the stick. Next day, in nearly the very same ambient temps (within a degree or two all day long), at the same travel speeds, same general terrain, the tranny ran about 15 degrees cooler! Now, that is not true of all trannies; only specific to the Ally 1000. But it surely contradicts what many folks think. Most would believe that more fluid would be "better", but it's not always the case. In my situation, being overfilled by two quarts made a big difference.
Originally Posted By: SteveSRT8
Wildly platform specific, some slushboxes don't care, others will grenade if overfilled...
What SRT said can be very much taken to heart; some trannies are nearly indifferent, others are very particular. You have to know your equipment, and be able to adjust accordingly.