Interesting that adding a MH helped... What exactly did it help? Did it just lower temps or did it reduce fatigue on the parts you noticed were already showing signs of stress?
Can't say that I know how, or even if, it helped.
On that one (a '10 2500) I foolishly didn't break it in correctly, and then outdid myself in stupidity and didn't change the fluid until after several thousand miles.
When it finally got on the lift, it wasn't a pretty sight. Almost all the paint had burnt off the cover and the "fluid" inside was black, and had lumps in it.
Can't recall what fluid went in, but the cover was cleaned and repainted. Only to find that after a 1,000-mile trip the paint was gone, again.
So it was in a bit of desperation that the MH cover went on, with Amsoil 75W-140, for the return trip.
Since powder coat holds up to heat better, I don't know how much cooler it ran, if at all.
What I do know is that the friend who bought that pickup still drives it, and has towed quite heavy with it, without any problems.