Lubrication Systems In order to maintain a constant oil pressure as the clearances between the moving parts of an engine increase through normal wear, the supply pump output a. increases as the resistance offered to the flow of oil increases with more oil being returned to the pump inlet by the relief valve. b. remains relatively constant (at a given RPM) with less oil being returned to the pump inlet by the relief valve. c. remains relatively constant (at a given RPM) with more oil being returned to the pump inlet by the relief valve.