If you do fill it up all the way it will only drip out a little bit when turned sideways to installed, maybe an ounce or two max
I've owned a bunch of early (pre-1972 ) A body small blocks with that oil filter adapter on them I started out half filling them and saw no oil loss at all when installing them, ended up filling them all the way up to the bottom of the steel adapter and lost a little bit of oil, that loss was worth having instant oil pressure after changing the oil and filters