Veicle has oil pressure, but oil guage doesn't register any. on 1990 Chevrolet G Series Van (G20)

Their is no hydraulic lifter noise and there is plenty of oil in the crankcase.

DID they give you the correct sending unit for your gauge and did anyone actually physically test the oil pressure with a regular shop guage at the port where the sending unit goes .ANYWAY yes they have different sending units for different gauge scales some go to diff. PSI ratings and the wrong sender may not send a 'valid' signal.just a thought ....actually I've got to go check the air pressure in my head .good luck.
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It is probably the sending unit. Located on top of the motor towards the back on the drivers side.
I changed the sending unit and it still would not register on the oil pressure guage. I think I have narrowed it down to the resistor on the gauge, but I have been unable to find one, even at parts stores.