Living in Arizona we see more than our share of serious mountains and lots of very hot days. I usually run near my GVWR and often tow my motorcycle {bike and trailer weigh right at 1,000# mas y menos}.
When I bought our 2012 E-350 used with just 6,205 miles one of the first things I did was change out the oil, trans and differential replacing all three with full synthetic. At 48K miles all is well, runs smooth and cool. I change the oil and filter every 5K and do the trans and differential every 30K. That may be over killl but as noted oil is cheap.
When I switched out the trans fluid from dino to synthetic it dropped my fluid temps almost 10 per cent across the board {you "do" have a Scan gauge to monitor your trans fluid temps.... right?}.
:C
