Im assuming your from the nj exit 117 area? Ifso, so am I , where abouts are you located? Im in middletown, I see you sated you have a gt500 I havent seen many of them around this area, I have a 03 cobra..
Yes, it's perfectly fine to not use 5W-50 in this car. I've already proven it's too viscous. Here is a word document that will make it easy to order Amsoil at cost, know what to order, and at what quantity.
Amsoil - GT500
If you have any trouble with that file, let me know.