Brent,
The Mobil 1, 5w-30 would be your best choice for warm to hot weather use. I'm not impressed at all by the M1, 0w-30 or 0w-40. Paradoxically however, their 0w-20 seems to hold up very well. I think I'd actually run the 0w-20 for the coldest six months out of the year and run the 5w-30 from April-Sept. Around here, "Advance Auto" carries the 0w-20 stuff for about $5.00/qt ....
Your equilibrium oil temps will run 10F-15F lower in the winter, so this will give you the same oil film thickness regardless of ambient temps ....The 0w-20 will also provide a fuel savings of approx 2% in comparison to the 5w-30, based on my own testing.
I think Ford has done their homework on the 5w-20 recommendation, I haven't seen any issues, even with the V-10 used in towing applications.
TS
