As I understand the oil pressure switch and dummy gage on the Explorer, the
oil pressure should read nominal when the pressure is greater than 6 psig.

So, you probably have a off-value resistor in the instrument cluster.

To check the real oil pressure, just pick up a mechanical gage at any auto
parts store for about 20 bucks and plug it in where the oil pressure sense
switch goes.

I'll bet you find the pressure is OK.  I think it should read about 10 to 20
psi at idle and maybe 50 to 60 at higher throttle settings.

Do you have a real pressure sender or just a switch? The real thing is much
larger, not just the tiny 1/4" thick, 1" diameter gizmo that costs $3 at a
parts store. If that's what you have, it's a switch. To the best of my
knowledge Ford stopped installing real pressure gauges many years ago 'to
avoid questions from users about the needle moving around all the time', and
they have been shipping the simple switch since then. The switch is off when
the pressure is below a certain minimum value (a few psi) and on when it
exceeds that value. A resistor in the circuit (behind the instrument panel)
sets the current at half scale when the switch is on, regardless of the
actual pressure. 10 psi or 100 psi, the needle remains in the same spot.
It's exactly as good as an idiot light, but makes their trucks look more
'professional'. If you have a switch and the needle goes to full scale,
either the resistor is shorted or the gauge itself (not the sending unit) is
bad. The switch is on the ground side, so defective insulation that you
suggested can only cause the needle to go to 'normal' when it should be
'low', but not anywhere higher. The good news is that as long as the needle
still drops to 'low' when the engine is off, you have nothing to worry
about, because the thing is doing its job and no one can tell what the
pressure is anyway.