Truck Repair: cummins n14 celect plus, cummins n14, scotch brite pad

Question
I am a certified tech but normaly work with Mack
  I have a cummins N14 celect plus in a Volvo with a history of killing # 5 injector after a very short period of time. and only 20 psi of oil pressure.  can you give me any leads? I am puzzeled as to why its always # 5 cylinder.

      thank you for your time

Answer
I have heard of that before on the same cyl but did not ask what fixed the problem.The oil pressure should be MIN 25 PSI at rated(MAX RPM),20 PSI at peak torque,and 8 PSI at idle.Anything below than will log a 143 fault code.If the oil is not diluted then try taking the regulator plunger out of the oil pump & clean the scratches of it with a Scotch-Brite pad.Then reinstall it & check it again.