Wind chill factor?

Pullman WA
I've got a 1992 Skylark V6, Buick recommends 10W-30 for this car, however under Zero degrees it recommends 5W-30. Where I live it might get under zero degrees a few times during the winter but not very often, but the wind chill can get under 0 degrees often. When making the decision should I take into consdieration the wind chill factor? I've used 10W-30 the last couple of years and haven't had any cold start problems (10W-30 Castrol).