kdoss: Is your head spinning yet from all the divergent opinions? ( : < )
Your truck will work very well with 5W20, towing or not. Ford has proved that over the past decade or so and if the engines were dropping like flies, we'd be hearing about it. I also urge you to start monitoring the oil analysis threads concerning the modular V8s. You won't find any bad UOAs due to 5W20 oil either, no matter what the operational situation. A 5W20 isn't always the right choice, but it's right 98 percent of the time and, I think, for you.
If you short hop it most of the time, the 5W20 is ideal because you are dealing with a situation where the oil seldom gets fully heated up. It takes about 15-20 mile of continuous driving to bring my 5.4L engine oil temp all the way to normal (which varies according to climate and workload.) Until oil reaches it's 100C (210F) viscosity temperature, it's running thicker. On a short hop, your 20 grade oil may spend the majority of it's time running in the 30 or 40 viscosity grade. Imagine what a 40 grade will do. Your engine will waste a lot of energy spinning in that thick oil and your cold oil flow is reduced. For the most part, neither of those things are terminal situations but they are far from optimal.
Also, a good 5W20 is robust enough to handle an occasional towing situation. Probably enough to handle a lot more than that. If you were to go coast-to-coast some hot summer with a capacity load, that would be the time to upgrade viscosity because it's likely oil temps would dictate a jump to a thicker oil. As the oil passes that 210F point, it begins to get thinner. Again, over short periods, and with reasonable temperatures, it's not harmful to go a bit over temp. It's been proven time and time again that there is plenty of safety factor built in.
Logic and efficiency dictates that if the engine spends most of the time running easy, why spend most of it running in a sub-obtimal condition with an oil thicker than it needs just to cover that small percentage of the time when you are running at the edge. Especially when the 5W20 can cover you during those times.
As to 64K on a well maintained engine... it's unlikely anything is worn enough to need a thicker oil due to increased bearing thicknesses. Those modulars stay pretty tight.
I also run a 5.4L, but its's the later 3V version. Mine is a working truck (on a farm). I monitor oil temperature and seldom does it exceed 210F, and then only by a few degrees. I don't need anything thicker and I don't think you do either. If you're worried, go back up to the 5W30 that's in your manual and sleep better.... but 5W20 will be fine. Maybe even optimal.