I have done Subaru diff covers (same cast aluminum against cast iron), plain Dana style steel covers, and dropout center sections with standard black RTV. I have not had a single leak within 30k, I have had a couple start to weep at about 34-36k though. I have also used the Ultra Gray RTV, and have not ever had a leak with that. I usually use Ultra Gray on gasketless oilpans, in between trans and transfer case, as well as timing covers. It's good stuff.

I just flushed the fluid in a 3/4 ton dodge rear axle today.
scraped all the orange goo that was on there from the last service, sprayed gasket remover and removed all residue. cleaned the cover, spread a 1/4" bead of ultra gray around the inner and outer area, and put the cover on. let it sit for 5 minutes, (instructions say spread and apply immediately) then filled with gear oil. no issues. ultra gray works wonderful and I just LOVE that pressurized can!!!