my owners manual calls for 75w/90 gear oil for the front and rear diff. is there any reason that you cannot use 80w 90 gear oil insted. the reason i ask is 80w 90 is plentiful. i can get it in gallon jugs, quart bottles whatever. 75w 90 only comes in quart bottle here and finding 5 in one store is almost impossible. just wondering what you guys think.