My 5.8 had a broken oil ring on #8, would foul that plug every 5k miles or so. Burned 1 quart per 200 miles or so. Tail pipe was black with soot. Every so often it would puff a black cloud like a rich diesel (and give a strange sound like detonation) but otherwise exhaust was smoke free. I ran it like that for a long time (didn't know what the issue was, tried lots of different stuff). Finally #8 exhaust valve had so much carbon deposited on it that it wouldn't close completely, and I had a persistent misfire with only 60psi static compression. The truck ran fine before the valve stopped seating, compression tests and leak down tests were fine. The upper intake had oil standing in it from all the blow by (it pooled up in the back of the plenum, I poured oil out when it was removed). The exhaust valves were all bright white except #8 (running lean) but I got great mileage for a 5.8L...I figured the OBDII system was seeing the oil as a rich mixture and leaning the fuel curves, but no so much it hurt performance (maybe not, but 18+mpg seemed too high!).
Try pulling plugs and seeing if there is one a few that have lot's of carbon build up, might help you figure out if its a burning issue or some illusive leak like others discussed above (page 1).
EDIT: I replied before reading pages 2 &3, sounds like this has all been covered. Also just realized the OP was in 04', hopefully he got it taken care of!
96' F150 5.8/E4OD 3.55 SCSB stockish