We're going into winter in Michigan, so I wanted to avoid 10W-anything. So it was down to 0W30 or 5W30. 0w30's almost always are going to be full synthetic, so more expensive than a conventional or synblend 5w30.
Honestly, you could probably use Mobil 1 15W-50 and have no starting issues down to about 0 degrees. It has been said that 1. The oil is just that good and 2. It is more like a Full Synthetic 10W and can perform very well that cold. The reviews of it on Amazon from users state this. They say it starts like a summer day, down to about 0 degrees F.
I imagine a 10W from Mobil 1 would be similar.
However, of course, a 5W or 0W would be as you say, no guessing.