Not sure what the guy at Mobil was saying. According to their website http://dallnd6.dal.mobil.com/GIS/MobilPDS.nsf/26b7c4b33367a4a086256665004e4266/951b10d046e0bafd852567ba0050ec5f?OpenDocument
it has some sort of friction modifier in it. I haven't heard of anyone having problems with Mobil 1 in limited slip applications. The only problem I see is that it isn't Formula 1 certified!