Any known problems with oil pressure? on 2001 Jeep Grand Cherokee

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Just bought used,before I picked it up owners had coil packs,valve cover, and a check engine light checked.They just paid for what the garage said needed done. I had taken it for 2 test drive and seemed ok before the work.W ithin 50 miles of buying jeep oil pressure dropped, changed sensor and no change in pressure reading was wondering if something could have happened when the valve cover was changed or of any problems with oil pressure that you could help with. Thank You

(2) Answers
How many miles on it? Is oil pressure OK when cold , then drops as warmed up ( more than a 20psi drop)? Possible sludge/debris in oil pick-up screen. And , yeah , checking with a manual guage to see what the real pressure is would be IDEAL.