BC at typical winter starting temps a 20 grade is tens of cSt thinner than a 30 regardless of 0W or 5WExactly what I was thinking.
Me, I worry more about fully warmed up viscosity, or I used to when I had 25 or 35 mile one-way, rush hour commutes.
I don't go out if its 10 below 0F or more. I've lost a few alternator bearings and one water pump doing so - on relatively new cars.
More things are lubed than the engine. Idler pulleys, wheel bearings, cv joints and differential too