This sounds like the classic Nissan problem of crappy oil pressure senders. I have 3 Nissans 300zxtt,patroland r33GTR. The same problem on the first 2 none yet for the GTR. If the guage fluctuations are random then I would look at the sender. If it was mechanical eg the pump it is unlikely to be random but rather consistantly low at low rpm or low pressure at high rpm. I have used VDO senders in the Z and it worked fine. Try a new sender before you go off and hang yourself over the cost of an engine rebuild. Part of the 32 problem is a small diameter oil pump drive on the crank. Problem was solved with the 33's. Good luck.
