Out of all of the Mustang's I've had, I have always used RP or Amsoil.
Now that I have a Track Pack GT and will be racing it, 5W50 is "required" however, I went with RP 10W40 since this is not going to be a track car and raced hard often. I've spoken to a couple of Engine oil guru's and they've flat out stated that the Motorcraft oil (5W50) shears too quickly easily becoming a 40 Weight oil, whereas an Amsoil or RP will hold up with harder driving.
In addition, I've always felt that Amsoil was a great oil and it had almost no oil consumption compared to the RP I used to use. Mobil one is most certainly one of the better options, but, after a lot of tests in which those results are posted online, Amsoil and RP perform better with less long-term wear on the engines.
2014 GT - Track Pack, Leather Recaros, Electronics Package, Back-up Camera - Ported CJ Manifold, SCJ Mono-Blade TB, Custom Intake, Kooks 1-3/4" LT's, Kooks O/R H-pipe, Borla S-Types, AED Tuned - 443 RWHP / 388 RWTQ
Ordered: 3/19/2014 - Built: 5/13/2014 - Delivered: 5/19/2014