In the mid-'80's Ford changed to an oil pressure SWITCH on the engine, so no sender.
The switch is "open" if oil pressure is 0 to 5psi, "closed" if oil pressure is 6psi or higher.
This is a 1 wire setup, the threads of the switch are grounded to block.
The switch is the Ground for the dashboard gauge, sender was as well but it was a variable ground, so gauge needle would move as oil pressure changed.
The switch means any movement of the needle is an electrical change not oil pressure change, outside of above or below 5.5psi.
Reving engine and seeing needle move means alternator voltage is changing, I am sure oil pressure is as well but that is not what you are seeing in that needle movement, it is an ON/OFF switch.
On the back of this type oil pressure gauge setup there will be a resistor, the value of this resistor sets where the needle will be when switch is a ground(closed).
The resistance in this resistor or the wire may have changed and needle is at a different location.
Or the power to the gauge is lower, corroded connector or wire.
So yes this can be "normal" as wiring and truck get older.
If needle was dropping to 0 at warm engine idle then that would be a concern, pressure would be below 5psi, you would usually get valve train "ticking" when pressure got down to 4psi or so.
Remove the wire from the switch on the engine, then ground that wire.
Turn on the key and and see where needle sits, if it is back to what you are used to then replace the switch.
If it is at it's "new" location then problem is in the wiring, gauge or power.
You can change pre-'95 oil pressure gauges to "real" pressure gauges by removing the switch and installing a PS60 sender on the engine.
And then removing the resistor on the back of the gauge.
In 1995 I think Ford built the resistor into the gauge, so not easy to convert.
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