X2 on redline. I filled my AX-15 with 10w-30 synthetic, GL-3, GL-4, and MTL and recorded temps on my tranny case on each over a week or so using an IR thermometer. From memory the MTL was about 15deg F cooler in my daily commute than any other fluid.... the full synthetic motor oil was definitely the hottest.
I was running around in 90-95F New Orleans weather.
I would definitely check your inner boot, those don't last long with the high temps. Also, the addition of louvers in the hood may cause the air under the truck to remain stagnant so although the under hood area is cooler, the transmission tunnel is actually hotter.