I just traded my '07 in a few months ago with 211,000 mi., trouble-free. Yes there were problem 6.0's, some right out of the dealership, but most issues can be attributed to power increases from "tuners".
Since its an '04 there is a good possibility that it has the round egr cooler, its bulletproof. The first thing that should be done is make sure it has the most current computer revision from Ford, and while its there have them test the FICM.
For oil I used 10w-30, year around for the last 130,000 mi. at 7,500 mi. intervals(I also have an older 7.3 IDI that also gets 10w-30). Synthetic oil is not needed for longevity for the engine or injectors. Fuel filter interval per owners manual is 15,000 mi., be absolutely sure that ONLY Motorcraft/Racor filters are used and that the oil filter cap is Motorcraft.
Racor holds the patent on the design and aftermarket filters have caused catostrophic damage to the 6.0, one aftermarket answer was to sell a new cap with a slightly different sized filter, when a new owner puts a Motorcraft filter with the incorrect cap the anti-drainback valve cannot function.
My 350 was used as you are describing this 450 to be used, don't fear using 10w-30 for heavy usage.