Fluctuating Oil Pressure

I have a 1991 Olds Cutlass Calais, with a 2.3 Quad 4. The issue that I'm hoping to get resolved is a fluctuating oil pressure. When driving under normal conditions the pressure is usually between 40 and 50psi. However, under no specific conditions the pressure drops to 18-20psi. Does anyone know what may be causing this drop in pressure, or could it just be a defective sensor/gauge? Thanks for all your help.

Welcome to AF, get a seperate mechanical gage on it and verify the dash readings, the only way to be sure what's what.