Have checked all the harness wires and connections from the oil pressure sender to the gauge idiot light. All appear good and ohmmeter shows no "rogue" grounding. However, on voltage checking, with the harness connected to the sender and the voltmeter connected to the gauge end of the harness (not connected to the gauge), with ignition on but engine not running, the gauge reads a full 12 volts (as it should?). when I disconnect the harness from the sender, the voltage drops to about 6 volts (instead of 0 volts?). Similarly, with the gauge connected to the harness, the idiot light is bright with the harness connected to the sender, but is dimly on with the harness disconnected from the sender? What could be causing this dim light error?
This effort was the beginning of attempting to discover why the idiot light is staying on when the engine is running with good oil pressure showing on the pressure gauge.

Just a WAG, I would say to check the Red/White wire on back of the gauges for 12V
or a bad connection, also check fuse connection on #9 fuse.

Is the voltage the opposite sign when it shows 6V? Could be something feeding back through the ground. Stranger things have happened.

--DD

Stumbled onto the problem. During previous ownership someone used a metal "sleeve" bulb holder instead of a plastic non conducting bulb holder, thus the power was grounding to the gauge cup. Replaced with a plastic bulb holder and problem appears to have gone away. Still need to see what the system does with engine running and oil pressure present -- next few days.