The 99-04 Ford OEM instrument cluster minor gauges (oil pressure, temperature & voltage) are designed to move only in extreme conditions rather than telling you exactly what is going on in your engine.
The oil pressure sensor is a 6 PSI pressure switch so the gauge reads somewhere in the normal range when the oil pressure is over 6 PSI. It reads low when the oil pressure is below 6PSI. In reality the oil pressure is always changing based on engine RPM and oil temperature.
The temperature gauge shows low when the engine temperature is cold, and it rapidly goes high when the engine is getting close to overheating. But the needle does not move when the engine temps are in the normal range.
The volt gauge also does not move when the vehicle's voltage is in the normal range.
If you are interested in having one or all of your oil pressure gauges read accurately, check out the gauge upgrade units below. (Note the bundle of all three gauge upgrade units at the bottom)