I an going to change the gear oil in my rear diff today or tomorrow. My manual recommends 75w-90 synthetic gear oil. The only 75w-90 that I can find is for limited slip diffs, I have a G80 Gov-Loc. I am looking at using Mobile 1 75w-90, amsoil, or Mobile Delvac (if I can find quarts of it. alternatively it is $47 a gallon) I do tow a boat very often in the summer, but not very far. I have read of guys going to 75w-140 because they tow. Does anyone feel as if this is necessary? Any recommendations on which gear oil to use? I have read some bad things about Lucas and Royal Purple. I would consider amsoil. But, can not find a place near me that carries it or that I can get a significant discount at. I can pickup 3 quarts of Mobile 1 for $22 after discounts. Any suggestions? I may attempt to get a hold of @1st Synthetics and see what his prices are for the 75w-90. Looking online I can only buy amsoil in cases of 12. - - - Updated - - - I am going with amsoil, I will return the mobile 1 I just purchased. I spoke with Scott from 1st Synthetics and his prices for guys on the forum are pretty darn good. So, i will be driving down to get it tomorrow.