Unbeknowing to me my neighbor/mechanic took the oil pump out of my old 454-gen4 motor and put it in the 502-gen6. Well no oil pressure when I started it. He's out of town so I took the motor back out and put in a new GM hi-flow pump for a gen6 and have 70 lbs. of pressure now. He swears the pumps are the same but I don't think so. Anyway now I have lifter problems so I think I'm going to give the motor back too him and get a new crate motor. What's the deal with the gen4 to gen5-6 oil pumps.