Yes, the oil pressure switch on the engine is either on or off, below 6psi is off above 6psi is on, so the gauge is 0 with engine off and just stays at 1/2 regardless of engine RPM, how ever if alternator changes voltage a bit oil pressure gauge needle will move a bit, lol.
It depends on if you want to just check real oil pressure or monitor real oil pressure.
The dash gauge on a '90 can be used if you pull out the dash and by pass the 20ohm resistor on the back of the gauge:
Pictures here: Ford Oil Pressure Gauge Fix
And then you will need an oil pressure Sender for the engine, Ford used the PS60($15-$20) until about 1985.
So that should fit your engine, it is larger than the switch so sometimes an extension is needed.
Current oil switch wire just hooks to this sender and with resistor on gauge by-passed you will have a real oil pressure gauge, although it doesn't have actual PSI numbers.
A Mechanical gauge uses an oil line from current switch location hole then thru the firewall and into cab, this would need to be mounted where ever you want, it will have actual PSI numbers.
Or you could use one of these types of gauges just hooked up temporarily in the engine bay to watch oil pressure while engine makes its noise.