Sometimes light piston slap sounds like a bad lifter tick. Thick, cold 20w50 will quiet light piston slap for a while, and piston swell when hot might make it quieter when warmed up.

I keep wondering where the air gets in. Is the siphon past the pump gears enough to suck air into the main/rod/cam bearing clearances? Or just the cam bearing clearance, near the lifter?

Even without an antidrainback valve in the filter, with a good film strength oil, I'd think that the oil system would stay primed, unless there is a real fat oil clearance somewhere, or I'm missing another source. Weak check ball spring in the HLA? Inside the little pin hole on top of the rocker? Anybody?