You'll be perfectly fine with a 5w-30 for barely below freezing. Switching to a 5w-20 from 5w-30 will NOT change cold weather viscosity for cold starts. Both those weights are still a 5w- oil, which means 5winter. The oil is a 5 weight when cold, and a 30 weight when warmed up due to polymers in motor oil. If you lived in a ridiculously cold climate up north, that's when a 0w-20 will come in handy. A 0 weight in the cold will flow really well.