oil usage?

Never any. And, yes, the conversion is 5.5 qts, but you normally shouldn't need more than 5 (may be too much even), since you'll still have oil in the block after draining main and sumps. Did it sit for a long time?
 
CHECK VALVE could fail , oil could be flowing into the bottom from the tank it happened to me
 
CHECK VALVE, hmm sound like something to look into. Tell me how you knew that's what it is and how to replace it I couldn't find it in the manual?