engine running the oil level must be at least 2" above the oil pickup, preferably more, along with that it must be below or away from the rotating parts, to some degree those dimensions can be changed depending on baffles, scrapes, windage trays etc, but for normal purposes the 2" dimension should be considered the minimum to prevent air being drawn into the pickup, the reason being in oil dynamics once air is drawn in it tends to creater a vortex ( bit like a reverse tornado or whirlpool effect) and once underway that effect is hard to stop until a wave of oil kills it.