I came across this site that sells a quart of Royal Purple gear oil for about $10 a quart shipped. Is this the best deal?

http://www.karlracing.com/html/purchace_gear_oils.html

I'm going to be using 75w-90 in the rear axle since I don't tow. What's the downside if I just went up to 75w-140? Loss of fuel economy?

 

your local napa stores handle royal purple but i think i would check into amzoil iuse it in everything i own also amzoil has the additive in it for suregrip if you have it