66 Cuda Oil Gauge

My oil gauge does not register when the car is running. Here's what I've done thus far. My conclusion is bad gauge but I know there are experts that can confirm this.

1. with switch on accessory or run before start the gauge pegs high
2. when I start the car is drifts to pegged low
3. when I disconnect the sending unit wire while running or prior to start it makes no difference
4. when I ground the sending unit wire the meter pegs high
4a. the sending unit wire is not broken or grounded based on my ohmmeter testing.
5. a new sending unit made no difference
6. a new circuit board for the dash made no difference (it was needed due to poor connections for other gauges)
7. All other gauges read correctly so I presume the voltage regulator in the fuel gauge is fine. The voltage seems to be what is expected ~ 5v to the oil and temperature gauges.
8. I've read and tried various tests for the sending unit no change
9. I just had the top of the engine rebuilt (head redone, new valves and guides, etc. and that mechanic assures there is oil pressure.

So any other ideas or should I just get the gauge rebuilt at this point?