I've been running royal purple 15w-40 full synthetic oil in mu 7.3 f250. After 6000 miles and a 600 mile trip coming up I decided to change my oil. So I changed it. Added 15 quarts of fresh oil (including the filter). When I drained the old oil back into the jugs, it only filled 9 quarts total. ...

So I'm thinking 5-6 quarts of oil low? Holy Crap?! What could it be? I get tiny bit of blue smoke slightly on acceleration. But that's it. No other symptoms I can see

 

A stock truck shouldn't get haze on acceleration... that might be your clue right there. If it were me, I'd do the Cody test to be sure - you can search that term here on the forum.

Is your fuel filter black?

 

I changed it about 5000 miles ago. Wasn't black. But I'll check again. My truck isn't stick actually. It has a Dp tuner. Straight pipe. And intake.

I run it on the highest setting, I've found to be the best power. The best fuel economy dor me too