For gear application, i think both are great with redline may have some edge on fuel economy and amsoil may have some edge on extended oci. For transmission, redline wins hands down with their specific offering.
Redline seems to put emphasis on their oil being LSD compatible, while AMSOIL asks to add additive for LSD. And after seeing so many posts saying AMSOIL reduced noise in their diffs, I just bought AMSOIL SVTQT 75W-110 for my rebuilt diff, which I should receive by next week.
I'm using Amsoil in the differential in my Ranger. The fluid was changed based on Ford's maintenance schedule, which said the original fill was good for 100,000 miles. The truck had 101,000 miles when I got it and the change was a few thousand miles later. (In other words, the previous owner obviously had not changed the fluid before trading in the truck.)
This is a standard 8.8" open diff with 3.55 gears, and the truck has the factory tow package. The gears looked just fine after I pulled off the rear cover to drain the diff.
In the past I have used Red Line products. Either Red Line or Amsoil will work just fine for you. What I like about Amsoil for this application is getting a better price as a Preferred Customer and the squeeze bag packaging, which made refilling the diff easy.
Amsoil in all my diffs since the mid 1990s. All Mercedes, Toyota RAV4, several BMWs, Wrangler, Blazer.
Just put Severe Gear 75W-90 in the front diff of the X3 today.
I'd love to report that it is miraculous, but the truth is; I don't know. I do know it comes out after 50k miles looking about the same as it went in. So in that respect, I feel better about it than I did conventional I've used.