http://www.carcraft.com/techarticles/synthetic_vs_conventional_oil/
Good article. In you live in negative 20 or 30's definitely want to use synthetic 0w-30
Don't use Castrol Synetec 0W30 (German Castrol or GC) if you're looking for a thinner winter oil.
It's actually about halfway between a 30 and a 40 weight.
Consider it a 0W35.
In otherwords, you're better off with a 5W30 or 10W30 than German Castrol, even from Castrol.
Summary: Oil grades mean nothing. Check the actual viscosity on the product data sheet.

