It would slurp oil. My 5.9 was using a quart every 600-700 miles. One day, pulling a loaded trailer, my truck started bucking and missing. I barely got home. I used a quart every 20 miles and it took me nearly an hour and a half to go the last 40 miles. You can open the throttle body and look in. It will be covered in oil if it's blown.

 

At 98k miles, mine didn't use a substantial amount of oil, but did have some oil on the plenum. Another symptom was that the van would usually not start on the first crank of the day, but would fire right up on the second crank.
Performed the plenum fix, and now it starts immediately upon first crank of the day. Vacuum has improved noticeably as well.