Something I learned with the Ford 4.6 motor is that it uses a very high volume oil pump compared to the pushrod motors it replaced. With 30W you will get ~90 PSI at a cold idle and ~30 PSI at a hot idle with a stock mod motor. Thats one of the (many) reasons a 20WT can work there and not on the older engines.
I would be inclined to go 10w30, but if the oil pressure got too high I would not do it. If I had one of these new Fords and had to run 5w20 then it would be Redline because it has a HTHS viscosity of a 30 weight.
After I put 4k miles on my new 2003 Mercury Gran Marquis, I changed the oil to a full synthetic 5w-30. I didn't feel comfortable using 5w-20 oil in this Texas heat, especially driving in stop & go traffic every day. The car now has 23k miles on it and so far no mechanical problems that I'm aware of. The car has never used any oil between changes. So far so good.
I use motorcraft 5-20 in a 04 Expedition with 5.4l. No oil usage in 5000 miles. Plant to stay with 5-20. Bought used 01 Merc Grsnd Marquis 4.6l changed oil to mc 5-20 immediatly. Used 1 quart in 3000 miles. Changed oil again will see if oil consumption continues. If so will go to a 10-30 oil.
Well I bought my car used but I still use 5W-20. I've used 5W-30, 0W-20 and 5W-20 and they all felt the same. So I decided to go with the one in the middle since it gets real hot in the summer and real cold in the winter.
FWIW the company I work for went 230,000 miles in an F-150 (1997) 4.6L with 15W-40 truck oil. Still ran like new but was in an accident and sent to the junkyard. My guess would be that it would easily make it to 300K with truck oil.