I run Mobile 1 in my diffs, I figure as rarely as I've had to change diff juice (once, and then only cuz I put on Riddlers) the extra cost is negligible. As far as motor oil I don't really see the benefit of synthetic vs. dino in my aplication. I change the oil regularly enough that extended life doesn't matter, and I have no performance mods beyond a CAI that would work the engine outside of it's normal specs, so the extra money seems a waste. Seems to me that when you start adding performance mods the need for better oil would be there.
"Experience is something you don't get until just after you need it." --Stephen Wright