What should the oil pressure be on an 1988 MK111?

[bruin]

Took my 1988 Mk111 in for an airconditioning problem. Whilst it was there the mechanic decided to fix an oil leak from the pressure sender. He installed an aftermarket and the oil pressure indicated in the lowest white band i.e. zero approx. after a turnpike run. Before the sender change it had been just under 20Psi. Took the car back and he installed a factory sender with the same result. The car runs normally no overheat, rattles etc, It would be very hard to get a direct reading gauge into the car (They bulit the car around the sender). Do I get an oil pump overhaul? or is this type of oil pressure normal for a turbo and my previous sender was reading high? The engine is a turbo and has 150K miles. Should I install the original sender?
Help! Bruin.