I want to put Amsoil Full Synthetic in my 2004 Cobra, but I cant decide what viscosity to go with. I live in Georgia so right now its in the 40-50, but in the summer it gets easily in the high 80s, sometimes in the 100s, rarely! Im looking at the XL 5W-20, XL 5W-30, ASL 5W-20, and the Signature Series 0W-30. I just want the best for my bone stock cobra with 31,000 miles on it. Any input please!