When I changed my front and rear diff. oil at I think it was 75K miles it looked suprisingly good, like I really did not need to change it. It was hardly discolored at all. I replaced it with dino oil. I heard but I don't remember where that it was best to use dino oil in the diffs. I also used regular non synthetic ATF fluid in the transfer case. I don't know if the synthetic oil is better in the transfer case or not.