Oil pressure develops when there is resistence to the flow of oil.Pump problems aside,low oil pressure can be caused by worn main,cam,and rod bearings,worn lifter bores, stuck pressure relief valves(block or pump mounted),clogged pickup screens,loose pickup tubes,or otherwise clogged passages.High oil temp, wrong viscosity,and low oil level can cause the problem as well.Pressure does not equal volume,however,there should always be at least 10 psi for every 1000 rpm.