Well wait, Tom, you are talking about Diffs or Trans or both? I believe the diffs take their own fluid, with some LSD in the rear. Your trans and transfer case take ATF. I'm running Mobile-1 all around in my diffs and trans/xfer case. Personally, I can't imagine it causing a problem if you were running synth and then just change to a different brand of synth, but then again, I am no expert.

My front diff is still seeping.

Bart