Synthetic or conventional any gl5 75w90 or 80w90 will work. If you're towing a lot or heavy like an rv, you can use a 75w140 instead. Use a bottle of friction modifier if you have an lsd, even if it says it's added already in the fluid you choose.
They're not too picky in my experience, just check/change it every now and again when it starts to look bad.