I am somewhat mystified as to the difference between Mobil 1 75W-90
(see http://www.mobil.com/USA-English/Lubes/PDS/GLXXENPVLMOMobil1SyntheticGearLubricant.asp )
and Mobile 1 LS 75W-90 (See http://www.mobil.com/USA-English/Lubes/PDS/GLXXENPVLMOMobil1_75W-90_SyntheticGearLubricant.asp )
Can anyone shed any light on the difference and which of these two products should I be using in the front and rear diffs. of my 2003HD D/A?

Good question. Have never seen two types offered here in the "States". Always just bought Mobile I Synthetic SAE 75-90 and use it in both differentials. There is nothing else marked on the bottle that I just looked at except a GL-5 rating.

Looking at the descriptions they're almost word-for-word identical too. About the only "real" difference I see is the last item in the Typical Properties chart, but since I don't know what that rating is for I can't say if one is better then the other. Wonder if a phone call to Mobil will shed some light on the topic...