I'm servicing my friend's 07 Trailblazer (4wd, 4.2I6) and would like to know what lubes are suitable for the front diff (rear for that matter too). I know the spec is the stupidly expensive GM oil, but will a 80w-90 (on a shorter service interval) suffice, or even a synthetic 75w-90? I realize the grape juice is a 75w-90 by the way. My main concern with the front diff is the fact that it's capacity is so small and I have seen several fail. If push comes to shove, I'll use the big $$$ juice, but would like to avoid it- especially if it can be circumvented by a shorter drain interval. Also, I read of several people having issues with the non GM lube in the G80 w/locker. If this doesn't have a locker/limited slip, then would an 80w-90 suffice?