I'm a retired truck mechanic, most over the road trucks have a transmission temp gauge. Reason being heat is one of the things that kills automatic transmissions. If the transmission is shifting frequently then it is being overworked and will heat up. The heat breaks down the fluid and that in turn causes excessive wear. Changing the AFT fluid every 30k miles will prolong the transmission life considerably. My personal belief is if you can run in OD and hold that gear you should be fine, I do know there are two schools of thought on this issue and I have technicians argue it back and forth.