I don't think you would see anything out of the ordinary.
I used my '03 F150 5.4L to tow my Jeep and trailer (6000lbs+/-)and a bed loaded with a welder, grinder, sawsall, tools, spare leaf springs, spare transfercase, extra trailer spares, etc. from Massachusetts to Utah running NAPA conventional 5W20 with a quart of Q-Torquepower 5W30 I had kicking around. The viscosity was around 8.8 cSt@100*C. Round trip in two weeks time was just over 5 thousand miles.
It ran smooth, no wierd noises, burps, or doo dee's. The Rockies layed the smack down too. It was to the floor at about 45-50 MPH when we reached some of those 11-12 thousand foot peaks. The turbo'd diesels in our group did fine though (need a Paxton...LOL).
I recommend sticking to the factory recommened viscosity. 5W20 for Ford, Dodge, and Toy's and 5W30 for GM.