I picked up an oil pressure/temp gauge assembly (The part number on the box is F6MZ-10883-AA) from a Ford dealer who was selling his excess parts inventory on ebay.
I took it out to my test set-up that I was using to check for fake gauges, and this oil pressure gauge pegs high when I ground the "S" terminal. With a 20 ohm resistor, it's about at midpoint on the scale.
The next thing to do is install it in the car, change the sending unit, and see what happens.
1997 Grand Marquis GS - Died
Now have a 2003 Grand Marquis LS