a good rule of thumb in this case is not to exceed 100 degrees over ambient temperature, never had a problem with any of my 1500 silverados. have a duramax 2500 now and the temperature doesnt change much pulling my puma… an example is 90 degrees outside, you dont wanna see tranny temps over 190-195 this is normal…. after 10,000 miles i switch my fluids to amsoil in the diffs and transmission, never had any problems….