Using the factory on/off pressure switch, the 2003-back oil pressure "gauge" (analog idiot light) is wound so it reads about 40% of full scale when grounded. If you try to use a variable resistance sending unit with this gauge, it can only read a fraction of that, restricting the needle movement to the very bottom of the scale.
So, I swapped in a 95 Ranger gauge segment, the only year that fits the 95~2003 cluster AND is a real gauge. But there is another problem. The original Ford sending unit from 1995 is very large and won't fit the 4.0L SOHC without remote mounting it. After a lot of searching, a G-E member found a Chrysler sending unit that will fit into the confined area near the 4.0L SOHC block BUT the pressure/resistance curve of that SU is way off for for the 95 Ranger gauge.
Here is the slender Chrysler sending unit and connector that will (barely) fit into the restricted area of the 4.0 SOHC block:
To make the 95 gauge work with the sending unit that would fit, I had to change the calibration resistor. This allows the gauge to register about 25~60% of full scale under normal oil pressure.
I made up this test rig to choose the calibration resistor value. There is the gauge, sending unit, 12V battery pack, variable resistance and a 0~100 PSI pressure regulator (not shown).
After this, it was just a matter of adjusting the needle to a reasonable position that would still allow it to return to zero when off.
As a bonus, the Chrysler sending unit assembly contains both an on/off pressure switch AND a variable resistance sending unit. To take advantage of this, I wired up a latching circuit using the on/off switch, a relay and a small Sonalert annunciator. It arms as soon as the start position of the ignition switch is selected. There is an immediate beeping alarm if the oil pressure goes away after that as long as the ignition switch remains on.
It has been correctly pointed out that it is more straightforward and probably easier to install a calibrated aftermarket gauge instead. I just did it on mine to see if it could be done.