it could be several things
     to much bearing clearence  
    if it a new engine make sure all oil gallery plugs are in
     stuck by pass on oil pump
     worn out oil pump
       just a few things to look at
                    Richard Hamilton

My first pinto had loose cam bearings. When it was cold the pressure was fine. As it warmed up the pressure dropped. Hot off the track the pressure at idle was 0 lbs. It took forever to find the problem.