I've never seen a clear plastic oil pressure line stay full, gravity drains the lines and the oil system galleys when the pump is not turning, correct? If so then air is suck into it and occupys the space in the oil galleys, correct? Once the motor is running the oil pushes the air out where ever it can and some of it always goes into the oil pressure line and gauge, correct? My point is air compresses and oil(most liquids) don't so the pressures will be the same with air in the line compared to no air in the line