'89 Chevy G30. 3 parts involved or just two? Gauge, Sending Unit, Sensor? Does this involve a process of elimination. If so, in what order? ☺️
If it was me, I would replace the sending unit before I did anything else. On a Chevy I think most are in the area of the distributor and probably pretty easy to get to with the dog house off.
EricTheCarGuy:
http://www.youtube.com/watch?v=I8teB16U0LA
You can also use a multmeter to help diagnose the exact problem once you verify you have good oil pressure.