PS60, or similar pressure sender is a variable resistor, vs a pressure switch sender which is on/off
They both use the vehicle Ground as the control, i.e. oil gauge gets 12volt with key on, if gauge is not grounded by sender then it shows 0 pressure
When there is oil pressure above 5psi, the switch sender is on/closed, grounded, so that ground passes thru the cluster's resistor and oil gauge gets a "partial" fixed ground, and gauge shows 1/3 to 1/2, depends on vehicle voltage 13.5 to 14.5volts
The PS60 is also a resistor, but resistance changes with oil pressure, low pressure is high resistance, higher pressure lower resistance, like a volume control or light dimmer
If used in line with clusters resistor the gauge needle most likely would stay on 0 since the Ground resistance would be too high to get any movement
If you REVed the engine to get higher oil pressure, so PS60's resistance value dropped, you "might" see some movement of the needle, but under normal operation it would just stay at 0
When Ford switched to the oil pressure switch in the late 1980s it was to stop the silly customer complaints of low oil pressure, and it worked
So after 6+ years they decided to stick with the switch(on/off) setup
And it was my understanding that in 1995, when the clusters were redesigned, the newer oil pressure gauges had an internal resistor, so no external resistor was used, so no way to "jumper it"
1995 was an oddball year for clusters, different than 1994 and 1996, so it may or may not have the external resistor, I don't think 1996 and up had it
Oil pressure is the oil the engine CAN NOT USE at that moment
You are seeing the pressure in the main oil passage as it builds up because the bearings and valve train can not use it, its BACK PRESSURE
Engines need a minimum of 5psi back pressure to over come gravity to keep valve train lubed, under 3psi you will usually start to hear "ticking" from valve train
Ford's pressure switch is set at 5.5psi, +/- 0.5psi
At higher RPMs more bearing cooling is needed so a higher back pressure is good, above 30psi is fine, anything more is just for "show", lol
Above 70psi and you can "wash" bearings, basically they can run dry because oil is spraying in/out too fast so is not coating and cooling the bearings, it turns into a "mist" instead of a stream, and you get spun and failed bearings