If it is losing prime when it sits it has a problem in the supply side of the system that is allowing it to lose the prime, its sucking air instead of oil on start up
Air is way thinner than oil so the oil pump will suck the easest fluid it can
I have fought that demon several times, it can be fixed
My 527 motor has sat a long time(4+ months) between dyno tests, it primes immeadeatly(SP?) every time I prime it before going to the dyno
BTW, it is a dual line system with the Milidon adapter spacer for the second line, another place for it to suck air