From what I've read the 5w50 is for vehicles being driven really hard all the time therefore it may be too thick for street use, and comes with some potential problems:
The Motorcraft 5W50 shears very quickly to a 40 grade; my 1350 mile UOA showed KV @ 100 degrees was down from a VOA value of 19.93 cSt to 15.06 cSt. The oil was from a 07 GT500 driven fairly conservatively.
Motorcraft and Castrol 5W-50 lubricants quickly shear to a light 40 grade after only 2,000 miles. I've seen two samples that were already at the heavy 30 grade viscosity after only 2,000 miles. I also haven't seen many UOAs from the GT500s using Motorcraft or Castrol 5W-50 that looked as good as my GT500 using Amsoil 20W-50.
My current fill is Amsoil 10W-40, and a UOA is just around the corner. I suspect this will be the winning lubricant for this engine, and also why Royal Purple suggests this grade exclusively.
Amsoil 20W-50, Amsoil 10W-40, and Red Line 5W-50 are true synthetic base stocks. No refined petroleum is present in the formulation. It's high in ZDDP (major anti-wear additive). Motorcraft and Castrol 5W-50 use a highly refined petroleum base stock. Lower in ZDDP