Hey Lemmy! First of all you would be better off with a higher quality synthetic like Amsoil Signature Series or Royal Purple HPS. They are both fantastic choices. I'll be linking a thread below that has all of the information you need for your GT500 when it comes to oil. Hope it helps, Have a good one.
http://www.svtperformance.com/forums/svt-shelby-gt500-150/787413-official-uoa-thread-oil-poll.html
http://www.svtperformance.com/forums/svt-shelby-gt500-150/787413-official-uoa-thread-oil-poll.html