Iron are cheaper and really to me when your talking 6000 pound truck I don't care if I'd save a lil weight with aluminum I know aluminum disapates heat faster still scares me mixing aluminum with cast iron jmo
I use the differential rtv mostly cause I like the grey ring it leaves around my pumpkin but same here goober up smash it on fill er up and keep on truckin
Almost sounds like your lacking fuel cooler air requires more fuel my snowmobile ran really good in below 0 temps and terrible at 30degrees come to find out po had jetted it and it was getting way to much fuel! Your problem sounds like the opposite