My 04 ford f250 with a 6.0 no oil pressure at start up and idle but when revved up to at least 800 to 900 then fine pressure. Very hard to start but will finally run

Verify oil pressure on Ford PSDs with a known good mechanical gauge. Oil pressure may be too low. The cutoff will shut the engine down when the oil pressure is too low. PSDs use the Hydraulic Electronic Unit Injection (HEUI) System, so it completely relies on the oil level and condition of the oil to fire the injectors. Running the oil level low or too long can cause unusual problems.

As the name implies, the HEUI injection system uses hydraulic energy to actuate unit injectors. A PSD has two oil pumps--a low pressure lubrication pump, located near or in the front engine cover below the water pump, and a high pressure hydraulic pump, located at the front of the engine V, underneath the fuel filter assembly. If the problem persists, focus on the High Pressure Oil Pump. If the Injector Control Pressure (ICP) is lower than expected (engine cranking), it is usually caused by low injection oil pressure or regulator (IPR) valve. High pressure oil is used to pressurize and inject fuel into the cylinders--each injector is essentially its own injection pump. The IPR is a by-pass valve that controls the high oil pressure, depending on demand. The ICP Sensor monitors the oil pressure in the LH head. Check for oil in the upper reservoir--it should be within one inch of the top (add as necessary). Verify that the correct oil is being used and that it has not thinned out. For no-start concerns, check ICP Sensor readings with a scan tool or pressure gauge.