oil pressure gauge question

Yes, the gauge itself can be off. The grounding of the wire is just an indicator, not an accurate test.

If you know what a resistor is, get a 22 ohm resistor and put it between the oil pressure sender wire and ground, and turn the key to RUN. The gauge should read about halfway.

Did you buy a sender specifically for this car?

Do your temp and fuel gauges read OK? If they read low too, then the problem may be in the voltage limiter.