What causes high oil pressure in a 1998 ford expedition?

Possible clogged oil galley, filter, defective oil pressure gage, or the use of an oil that is the incorrect weight. Replace the oil filter with a Purolator filter and change the oil. Install 5W-30 oil. If the pressure is still high then you need to have the oil pressure tested with an accurate test gage, to see exactly what it is.