Oil Pressure Gage and Sending Unit Revisited

What have you done to the cluster. Think in terms of the entire path

Someone already mentioned grounding. "Rig" a wire between engine block--your meter---and the cluster ground. Should be zero "drop" between the two

On the hot side suspect anything. How tight and clean is the sender to wire connector? Possibility of poor connection in bulkhead connector? Or did you do the test resistors right at the engine sender connector?

Another difference:

You tested gauge, was it running or stopped? "It might be" that with system running at charge IE 14v instead of 12....something., that the limiter output is going down instead of up, as it may be cycling differently