If a lifter popped out of a bore, I'd be inclined to think it's run like crap. Runs like a stalled dog when It has pressure and I can actually drive it. This problem has been on and off for a bit. Maybe the last 400 miles or so.
I agree absolutely, it should run like crap.
It's a possitive displacement pump, with atmopheric pressure pushing down on the oil, the pickup can be 4 inches off the bottom, as long as it is submerged, atmospheric pressure will force oil anywhere there is lower pressure.
IF the pump is turning, the oil is going somewhere.
You didn't mention if there was oil in the filter.
Any air sucked into the pump will make oil frothy/foamy.
The volume of oil lost suggests an open galley.
Ergo, - easiest is lifter, it gets tuffer after that.
My $ .02
Good luck
