It's probably because most mustangs take 5w-20. I use to put 5w-50 (motorcraft) until I realized a "ticking" coming from the engine every time they changed the oil. I switched to 0w-30 or 0w-40 and the ticking has 99% gone away and the car accelerates a little better. Also gas mileage went up a little too.
Thinking about it now, a 50w oil is a little ridiculous if your driving on the strip or stip. Also, did you ever notice on the new 2015 track pack (performance pack) mustang they recommend 5w-20. In fact all mustangs are now suppose to be running 5w-20.
In conclusion, I feel that 5w-20 is fine for most driving, but if you go to the track, participate in HPDE events, etc. you should run something thicker.