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There is probably no oil pump problem as the engine runs but has not been started in "several/few years" ...The oil could have changed consistency into a sludge situation from lack of use....If you cannot get pressure by driving the pump from under the distributor described above, draining the oil may give you a clue...at all costs dont run the engine with out pressure....
this might be a dumb reply/answer, but is there oil in it?.....
Have you checked the wire from the sending unit to the guage? Also, as has been said before, you can put a mechanical guage on it, temporarily, don't mount it, just leave it under the hood to see if you do in fact have oil pressure. Chances are it's the wiring or the guage.