I don't have a '90 300zx in front of me at the moment, but most
engineers design gauges so that "normal" is at the halfway point. A
little high or low is OK, but a big variation is cause for concern.

Low oil pressure readings can mean a worm engine or a weak oil pump.
High ratings can mean clogged passages in the engine. All this
assuming you've changed the oil relatively recently and put in a
proper filter.

There may also be more specific things a 300zx gauge tells you - for
example, a reading that varies significantly with engines RPM
indicates wear - but what specific wear is typical in that motor would
be better found by seeking out a web forum specific to Z cars.

I don't have a '90 300zx in front of me at the moment, but most
engineers design gauges so that "normal" is at the halfway point. A
little high or low is OK, but a big variation is cause for concern.

Low oil pressure readings can mean a worm engine or a weak oil pump.
High ratings can mean clogged passages in the engine. All this
assuming you've changed the oil relatively recently and put in a
proper filter.

There may also be more specific things a 300zx gauge tells you - for
example, a reading that varies significantly with engines RPM
indicates wear - but what specific wear is typical in that motor would
be better found by seeking out a web forum specific to Z cars.