oil pressure when engine off

The question is, "What Sensor do you have?"

Rotax has changed the sensor over the years.

If it a 3-wire sensor, it has a 4 to 20mA output. 0-10bar (150psi)

If it is a 1 wire sensor, it is resistive.

Whichever you have, pulling the connector at the sender should produce a ZERO Reading on the panel.

With Dynon and if it is a 4-20ma sensor and if the current drops below 4ma as the connector is unplugged Dynon will display a red X over the gauge. Anything under 4ma is read as invalid. If the sensor is engine mounted it has probably been damaged by heat and vibration. 

 The sensor is brand new, never used, latest Rotax model, 3 wire.

Well first unplug the sensor and let us know what you find.

A Zero or RED-X likely means the gauge and wiring are OK.

Do you have access to a KNOWN-GOOD sender?

Next, how involved are you willing to go to resolve this issue.

Do you just want to throw money at the problem and get it fixed quickly without ever really knowing what the problem was?

Or are you willing and able to spend some time building a couple of basic test fixtures to test the gauge and senders individually?

Bill is right. The minimum reading is a feature and proof that the sender is alive and good.

In case your display shows  0 one day something is wrong and needs checking/fixing.