Honestly whatever I have or have left over.... currently it is walmart brand 80/90 mixed with a little 75/140
imo, you should be using the very best gear oil you can get your hands on. ARBs are expensive, and high quality fluids are cheap insurance to keep it functioning smoothly and properly. the same goes for any locker. i use Mobil 1, but there's also Royal Purple, AMSoil, whatever... people will debate which is the best oil and i don't want to get into that argument. but i doubt that there are many people who will argue in favor of walmart brand or any other cheap oil for an ARB.
mixing viscosities probably isn't the best idea, either.