Have you changed the Oil Pressure Sending Unit?
Do you have a Quality Oil Filter installed?
The 1987-91 instrument panel Oil Pressure Gauge are known to read incorrectly.
I have seen these items indicate low oil pressure.
Does the engine knock on start up?
If it does and you replace the oil pump, while you have the engine out and the oil pan off; install main-bearings on the crankshaft.
Oil Pressure: 35-40 psi for the 4.9L at 2500 RPM / normal temperature.
Hope this helps.
Last edited by 1987 F-150 XLT 4x4; 03-05-2010 at 06:47 AM.