5W20 is the recommendation; in 94k miles, my 4.6 has lived on 5W20 for its entire life, consumes no oil, makes no noises, and runs wonderfully. I would stick with the factory recommendation--although the heavier oil may be OK in a pinch in warmer weather/warmer climates, the valvetrain on the modular motors needs oil STAT upon startup or it will wear prematurely and cause problems.

Good luck for a long life from your new-to-you Ford truck.

George