Sudden Loss of Oil Pressure

There's probably small burr or debri on the oilpump internal pressure valve or bore, keeping it open. There's still flow, but hardly any pressure.

Or a lifter has jumped its bore, bent pushrod, acting like a big oil leak now internally. But should be very obvious cause engine will idle rough like missing a cylinder.

Pre-lube the engine through the distributor hole after removing the intermediate shaft. You will right away if it's building oilpressure. (Rotate counter clockwise).