Interesting, thanks for the links. I wonder what temperatures Mobil consider 'very cold'?
So yea, in that case anyway, a thinner oil will mean the bypass valve closing earlier at the critical cold temperature ranges, (whatever they are!
[edit] A short look turned up nothing, other than 'cold'
). So 10w/60 would be affected more than most other oils, bar the old 20w/x oils, lol.