If U want total pinpoint accuracy, U better install a quality aftermarket guage..
The OE guage is fine for approximating the amount of oil pressure, will indicate the typical fluctuations when the oil is cold or hot as well as increasing with RPM..
In my 70 RR, I have both the ralley dash oil pressure guage and a mechanical guage mounted directly to the engine, not piped inside.. This gives U the best of both, OE guage for basic tracking while driving and EZ access to the mechanical guage for more accurate pressure readings/troubleshooting/monitoring...

Just my $0.02...