In the manual of my 2005 dually it says to change the auto transmission fluid and differentials every 30,000 miles. If you take it to a dealer they will do a transmission flush rather than just a drain, price around $125 is what I have paid. However I use a speed shop. Not sure how they hookup their equipment but I feel the flush is necessary because I haul for a living. The truck is loaded about 60-65% at an average of 20,000 pounds.
I did make the comment to the shop the first time that my torque converter has a drain plug. Their comment was that it wasn't necessary. They charged a bit more than the dealer but I feel they treat my truck better than a dealer.
My Sun Coast transmission was installed at about 90,000 miles and now the odometer is 387,000. It still shifts the same as the day it was installed. I attribute this to the preventive maintainance and of course the quality of the Sun Coast rebuild.
I might add the manual also suggests flushing the cooling system every 100,000 miles.
I use Delo 400 every 10,000 miles drive at 58 mph when loaded, usually 65 sometimes 70 empty and got 161,000 miles from my last set of Michilens.