I know this goes a little against the "spoken" Amsoil policy, But CERTAINLY NOT against the written policy - ref brochure G52, page 4, chart in first column:

It really depends on your outdoor lowest winter temp and your highest outdoor summer temp.

Synthetic 0W-30 is still a 30 weight oil at over 100°F outdoor temp and will offer great protection for average passenger vehicles in even the hottest places in the US and yet flow perfectly lower than -30° F.

It's the EXTREME abuse turbo cars that need perhaps a lower SAE grade spread (not to be confused with viscosity index) and a higher SAE "summer number". Of course, again, that winter number is very important for cold starts for all engine parts as well as the turbo. On paper, (same chart as referenced above) a 20W-50 is good down to 0°F - but I like a hefty margin of error that's why the 35-40°F bottom recommendation from me.

I must admit on a personal use note - yesterday AM we had an early spring frost - clear as a bell but air temp was 35° F when I started the car. Oil pressure came up instantly. DO NOT try this with petroleum 20W-50!!!!!!!

I guess to finally answer your question I would pick 10W-30 for it's low spread, -20°F winter protection rating, and way past 100°F summer protection.

If you or anyone else would like the "G52 Motor Oil" - 8 page brochure, with all sorts of great info. shoot me an email with your address. Great reading even if you aren't interested in Amsoil.