Nissan gauges are more like guidelines than rules. First of all, check the connector at the oil pressure sender on the block. I have found on my RB30 that the spade connector goes slack over a period of several months and I get a low reading. I just slide under the vehicle and use a pair of pliers to squash the lug a bit tighter and the reading jumps a couple of bars. Other than that, check the earth straps to the block. Remove them and clean the connecting surfaces. It won't hurt to add earth straps from the block to the chassis and body and from the trailer socket to the chassis.
If that doesn't fix the problem check the earths on the dash cluster.
On high mileage engines using a slightly higher grade oil sometimes raises the oil pressure too.
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