From what I've heard/read/experienced, it seems to me that exactly which Mobil 1 viscosity one chooses is really not that important. The data sheet says it all. For the vast majority of us, pumpability limits are so low even for 15W-50 that even it would work just fine, as verified by Wisconsinites and Chicagoans I've heard from.
I second the remarks above about dedicated winter tires. TireRack can send you mounted and balanced tire/wheel sets with Michelin Arctic Alpins (my choice) or Blizzaks.