Time has come to replace the rotted differential cover on the S10, as with it I need gear oil. Previous owner replaced the stock internals to a Detroit style locker, not sure what brand it is. Pretty sure they didn't spring for the Eaton brand. I've heard not to use synthetic and no friction modifier. Also have read that a 75w140 will help quiet down all the popping, banging, and clicking. Not worried about economy as it might get 15-20mpg on a good day. It will also be daily driven in the winters around here. What do you guys think, stick with a 75w90 or try the 75w140?