Edit, this reply was to goodtimes last post...
That's what was strange...I'm used to hearing a few dull thuds on occasion upon the first start up with the new filter..most times its quiet. Oil can drain back through 2 channels...through the bearings, which is a given, and will happen...or back through the oil pump gears or whatever pump mechanism exists...which is usually a fairly tight interface, since after all...it has to act as a pump. So long as the pump stays primed..oil pressure will recover very quickly upon startup. If the oil pump loses prime, it takes a bit more time.
When I change a filter, the old one is removed and the new one screwed on...I thought about it for a bit, and it's rare that more than 20-30 seconds would lapse between removal and installation. Regardless as to whether or not the filter has air or oil in it, the ADBV (if it works right) will create an air lock in the system and prevent further drainback through the pump...much like holding your finger over the end of a straw and pulling it out of a glass of iced tea. The pump, in 20 or 30 seconds, shouldn't allow much oil past, so it should prime fairly instantaneously, and usually does.
Even with the [censored] filter on my 4runner, I could crank it and turn the key back to off before it fired, then on the second attempt to start, it would start with no noise. If I allowed it to start on the first attempt, it sounded like something wanted out.