Is your sending unit a mechanical or electric? IF it's not mechanical then I would run the motor with a mechanical to be sure. Electric ones are never really accurate, and if your sending unit isn't the correct one for the gauge, then your reading will be way off. RacingRascal's oil pressure gauge read zero when we took the boat out but is was the gauge that went bad, and it was brand new. Check these things first before tearing apart the motor.......and still keep your fingers crossed.
HBjet