The car is a Mazda Miata. Owners manual calls for 90wt. gear lube in the diff. I can't find any straight 90wt. lubes. So the question is can I use a 75-90wt. lube? Mobil 1, Valvoline etc. make a synthetic 75-90wt. and a synthetic would be my preference if that weight can be used. What do you guys think?