1998 Jeep Cherokee - Low Oil Pressure

Have a 1998 Jeep Cherokee 4.0L with 278,000 miles. Installed a rebuilt motor 45,000 miles ago. Normal oil pressure indications prior to installation of rebuilt engine. With rebuilt engine pressure was between 30-40 even when driving on highway. Mileage remains good and no overheating was observed on the dash gage. Recently oil pressure has dropped to between 10-20 with check engine light coming on at idle. Oil pressure indication from panel gage never rises between 20-25. When starting cold, pressure runs between 40-50 for about 10 miles then gradually begins to drop. Recently had to replace fan motor. Have replaced oil at regular intervals with no excessive loss observed. Have replaced sensor on engine block and low oil pressure indication continues.