Also from the Workshop Manual:
Originally Posted by Ford
Pressure should not exceed 5 bar at speeds over 2000rpm (using 20W-50 oil)
The following typical faults can occur:
Pressure too high at speeds above 2000rpm:
Pressure relief valve failing to open due to dirt
Pressure too low over the entire speed range:
Oil intake strainer clogged, intake pipe loose, or broken, oil pump worn etc.
Pressure too low in the lower speed range:
Pressure relief valve sticking in the open position due to dirt.