is the severe gear 75w90 oil from Amsoil interchangedable with the SAE80w90 that the manuel reccomends? keep in mind that i have a locker. i want different oil cuz the stuff i have in there now doesn't really keep the diff very cool.

 

Yes, you will be fine. I would however run the 75w-140 if you are going to be towing a lot...

I run 75w-140 in my jeep (front and rear axles) and it has made a noticeable difference. Both in temp, and in cold weather performance....

 

In my manual it says use 80w-90, the tag on the axle reads 75w-140. Pick your choices.

 

I noticed a little hit in MPG when I changed to the heavy stuff. I use
80-140 red line synthetic. It really quieted down the lockers though.
I was using 75-90 before.
I think any quality gear oil especially synthetic will help keep it cooler.
If you are really worried about it, you could get a new cover with either cooling fins or better yet increased fluid capacity.