maybe the RB gauge adapter or something like it?

 

unscrew stock sender, put mechanical gauge in there, check to see if it performs to spec, and go from there.

there are several things that both control oil pressure, and measure it. its usually a good idea to narrow down the culprits, in this case to the gauge side, or the actual oiling system.

the gauge is easier, as we can put a resistor on it, and see where the needle goes, and then we can check the sensor on the engine the same way.

if that doesn't work, next step is to verify the oil pressure in the engine, mechanical gauge, screws in place of the stock one. if oil pressure is STILL not right, then oil pan comes off, and there are two regulators.