Oil pressure gauge reads zero after 20 minutes

sounds like oil pressure sensor is bad to check the oil pressure remove sensor and screw in oil pressure gauge if the engine isnt making a noise with oil pressure reading 0 then it is your sensor that is bad

do a manual oil pressure test to determine issue

Is the engine making any ticking or tapping noises? If not, then it is your sensor or guage. Screw in a manual guage in place of the oil pressure switch and get a reading. If there si no noise, chances are it'll read normal. To test the guage you need a variable resistor box (guage tester). Easier to just replace the sending unit and see what happens first.