Welcome to the forum!
as far as the Mustang goes, nice car! Also, as long as the garage fills out your maintenance info, you shouldnt have a problem upholding your warantee.
I would look for a reputable private shop and take it there. They will usually take more care than the quick lube places.
If at all possible, I would do it yourself. Its an easy job, a lot of guys on this site could walk you through it.
As for oil, I believe the mustang calls for a 5w20 in the V8.
If youre looking to drive it like it was meant to be driven, and live in the Vegas heat, I would run any good quality synthetic.
Mobil 1 5w20 is good, so is pennzoil platinum and many others.
Amsoil makes some nice 20 wieghts, but they can be a bit spendy.