The oil pressure gauge in the Sport Trac's is not a true gauge...It's a form of Idiot Light. The Oil Pressure sensor is just a pressure switch that turns the gauge on and off. That's why you do not see the oil pressure change with different engine RPM...If the sensor gets 8 PSI or more of oil pressure is switches the gauge on and the sensor nor gauge will ever vary from that position....ie: Idiot Light.
The oil pressure sensor switch is a known problem, especially on the early Gen1 Sport Tracs, and it tends to happen when the weather starts to get cold and the overnight temperature drops into the 20's or low 30's.
As previously stated...simply replace the sensor, or just ignore it and within a minute or so the sensor will pop on and turn on the Idiot Light Gauge.
...Rich