Extra capacity transaxle covers

I decided one time to keep track of the rear differential temperature to judge if mor cooling would help. I have a well overloaded GMC 3500 dually. After 4 or 5 days of running in moderate temps of about 80F the differential never got too hot to touch which for me is about 120F. Extra capacity for that is a waste of money. Now back in about 2003 or so Ford changed their differential cover from steel to aluminum with fins for cooling. They must have had some problems for them to spend the money for a cast cover. I like to use Redline Synthetic, expensive but sure has solved some transmission problems on other cars.