Hey Buster, been a long time . Just a quick post then off i go but >>>>
Looks like Mobil is using a different dispersant/ detergent pack , similar to what Maxima has been using in their <10w-40 Extra> for years now. Lower starting TBN that hangs in longer and all that goes with it, less the viscosity of course.
Perhaps starting vi @100c will trend lower as well as phos amounts are lowered.
Sulphonates out-Salicylates in ?