No need to go crazy trying to find a SAE90 gear oil. Any of the 80w90 and 75w90 GL5 gear oils will work fine. If you want a little better protection, use a 75w110(Amsoil, Klotz JoeGibbs), or blend in some 75w140 with the 75w90.
RoyalPurple, Motul,.... still have a SAE90 for those that don't understand. And, I think that you can pick up some Diaqueen SAE90 (overpriced gallon) at the local Mitsubishi dealer.