The cam timing works fine with a variety of viscosities. Ford probably uses a 5w-20 oil to get a little better mpg in the PU's that the V10 is used in.
I've put a lot of miles on my V10 with either Mobil 1 15w-50 or 5w-40. Works fine and provides better lube strength when climbing a long, steep grade in 90 degree weather pulling a toad.