I use the same oil year around. The upper number is the operative one when the oil is hot. I like the 5W-40 number spread (Amsoil in this case), but I've used M1 5W-30 too. Nothing wrong with that.
The lower number kinda goes with what your personal philosophy about oil is.
Believing oils are much better than they were even 10 years ago, I think a lower low-number flows faster and better on a cold start. You may believe a higher low-number coats parts better for cold-start protection.
I don't think you can go wrong either way, as long as you use a synthetic.