A 25,000 mile oil seems to good to be true. If you read enough at the site it says 3 times the owners manual oil change interval for turbocharged cars. That is 9,000 to 15,000 miles on a GT. To be safe with your waranty 5,000 miles is all you can go. Amsoil can be hard to find. Mobil 1 is everywhere. Amsoil costs more than Mobil 1 ($4.09 at Wal-mart).
I just did my second oil change today at 4,000 miles and put in Mobil 1. The engine seems quieter and smoother.
Check the forums here for good oil info.
http://www.bobistheoilguy.com/