It's a mechanical gauge, meaning that pressurized fluid (oil) actually flows into the gauge when the engine is running. Perhaps a bit of trash in the oil got into the gauge and is affecting the display (assuming that the gauge is registering 20 when the pressure is actually zero?? I've made mechanical gauges work before by cleaning them inside *using alcohol or something similar*, then regulated air pressure into the gauge to test it as various pressures. If you have an air compressor with regulator, simply adjust the pressure to a low pressure and hold the air pressure into the gauge (an air blower with rubber outlet is good for that)... vary the Air Comp pressure over the range of the gauge and test it's accuracy. There is very little that can really damage one of these mechanical gauges.
