I would not recommend running any oil over 5,000 miles in your Powerstroke without doing a UOA (used oil analysis) to establish some historical records. You don't want to start running extended oil drain intervals without establishing a baseline and determining how the oil properties are holding up as you increase the drain intervals over 5K. Synthetics can be run 10K+ in some applications but the Powerstroke's HEUI Injection system is harder on oil. With a chip in a 7.3L you can easily hit 3000+ ICP and 7 times that at the injector - even higher in a 6.0L. It is quite common for 7.3L's with some Dual HPOP setups and 6.0L's to shear the viscosity down to a 30 weight oil before 5K.
I'm not saying that you will always have to do a UOA, just do them long enough to establish a history and at a point to where you feel comfortable with the results.
Not knocking Amsoil - but the Powertroke HEUI Injection system is much harder on oil than your brothers Cummins. You can do a search here or on http://www.bobistheoilguy.com
and see some Used Oil Analysis reports running Amsoil in this application.