From a simple google search here are the possible causes on a river V8, if not the gauge, with my thoughts in CAPS....
Common Causes
Air Lock/Lost Prime: The pump cannot pull oil because of air in the system. WAS FINE AFTER OIL CHANGE SO NO AIR COULD GET IN
Stuck Pressure Relief Valve: The plunger sticks in the open position, bypassing all pressure. POSSIBILITY
Worn Oil Pump: Scored or worn oil pump gears on the front cover, which is common in higher-mileage or older engines. UNLIKLEY ENGINE ONLY DONE 10K MILES
Blocked/Damaged Pickup: The oil pick-up screen in the sump may be blocked or sucking air. UNLIKLEY AS WAS WORKING FINE WHEN PUT AWAY
Incorrect Gasket: Using an incorrect front cover gasket or sealant can cause air leaks, preventing pressure build-up. UNLIKLEY AS NOT CHANGED
Appreciate any other suggestions for advice on next steps
