So switched off immediately. Question: Could there be a reason
other than a failed oil pump ? Oil level is fine. Oil and filter was
changed relatively recently.
Truck was running fine before this.

Sure, a failed sensor switch. Put a mechanical gauge on it, if the pressure is normal then the problem is in the switch, wiring, or cluster. You can vindicate the wiring and cluster by yanking the wire and then grounding the wire. Removing it should extinguish the light, grounding it should make the light turn on. If so, the circuit and cluster are good.

Thanks for the tips. Seems easier to just replace the sensor.
We'll see how it goes from there.