Castrol synthetic or Valvoline?

I would run valvoline synpower over Castrol syntec and I would run 5w-30 over 10w-30 in a 360 magnum.
I'm running Mobil 1 0w-40 in my 360 Magnum, prior to running the 0w-40 I was running valvoline synpower 5w-30 and it was great oil, ran nice and smooth and quiet on the synpower.
And synpower puts out impressive uoa's.
Only reason I switched to 0w-40 is because I live in Montana where winter is the longest season of the year where it routinely gets well below zero for weeks at a time.