it's a diesel, and they are frequently referred to as "oil burners." mine hasn't used a quart of oil in over 80k i've put on it (115+ total), however that's still what i call it. older ones tended to burn oil, a lot. most of the big trucks at work use quite a bit of oil, in fact, and those are mostly 03-07's.
anyhow, the one time i've had somebody else change my oil a couple years ago, he charged me $120, and that was paying him what he paid for the oil from WM (he's got a shop, however he doesn't carry rotella syn, and that was the first run i made with the stuff). like i say, oil + filter cost me $85.
the book says 15 quarts, however i started running 14 a long time ago. it shows toward the top end of the full on my dip stick and since it hasn't used oil (knock on wood), what's it going to hurt? i know a variety of other people who run 14 quarts.
for more info on the 6.0 specifically, visit the info center i use most often... 6.0L Power Stroke Diesel - Ford Truck Enthusiasts Forums
and for the record i've changed the oil since i got it and had a turbo replaced, ball joints/wheel bearing, FICM, clutch/flywheel, and i think that's it. all under warranty at a total of three different dealers. nobody complained. if you have a dealer that's going to pitch a fit, i'd find another dealer. these things are complicated enough that it's WORTH it to find a GOOD dealer/technician, even if it means driving further. i'm lucky, i've got a good 6.0 guy less than 40 miles from me (closest sizable town).