low oil pressure

my 1994 jeep grand ltd 4.0L has had a low oil pressure problem ever since i got it 2 years ago. it runs at 20 psi when it should be at 40 psi and when your foot is on the gas 40 psi instead of 60 psi (what happen to the 20psi of pressure). if anyone could tell me how i can fix this problem that would be great. Also it knocks really bad, but i dont know if the problems are connected. the jeep has 130,000 miles if that helps?

Someone can correct me if I'm wrong, but I believe the specs are that they system can run as low as 13ps at idle when the engine is hot.

Otherwise low oil pressure can be caused by two things:

1: Bad oil pump
2: Wear on the crank bearings.

yes thats correct. Run a thicker oil in it and if that doesnt make it move then suspect a faulty pressure sensor.