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).