An "at temperature" 50 wt oil is much thinner than a cold 20 wt oil. Temperatures in the turbo bearings will nearly certainly be hot enough to induce coking in a lower-standard 20 wt oil. The Amsoil 5W50 is a great choice, and it is plenty thin at operating temps to properly lubricate everything in your car and provide some cushion to ensure it doesn't eat itself alive due to thermal breakdown of the thinner oils.
What viscosity does the turbo manufacturer recommend? If they say 5W-50 is fine, you could just stick to what you have been using.
I'm pretty sure that the 2015 and up GT's all spec'd 5W-20 whether it was a track pack or not so who really knows if the 5W-50 was really needed. The twin turbo ecoboost trucks recommend 5W-30 so you would probably be fine with that.
wtd, the Ecoboost was designed and tested with the twin turbos, and engine management and cooling capacities were engineered from the start to deal with the heat and requirements of the twin turbos; the Mustang was not. If it was my car, 5W50 would stay in it. No harm in being a little cautious; lots of harm is possible if the choice is reckless.
High HTHS 40 grade true synthetic short drain oil should be the most you need.
You are not going full boost for more than 15- 30 seconds at a time - I will assure you.
Just keep an eye out on the oil sump temp and pressure gauges.
Other stuff is likely to break first.
I hope not much else's breaks, pretty much everything has been upgraded lol
I went with the 5w-50 as usual, ordered it and the diff fluid and the BG synchroshift II for the trans yesterday. Thanks to everyone for the suggestions.