I bought a 2001 GCL 95000 miles it runs great but after about 20 minutes the oil pressure drops to about 5 I put a new sending unit in but same thing so I hooks up a manual gauge in reads low after it warms up 4.0 motor I thought these were very long lasting motors any ideas could It be oil pump

 

03 FSM says it should be 13psi at idle and 37 to 75 psi above 1600 rpm. They are long lasting when properly maintained. If you have less than 13psi then you need to drop the bottom end to figure out why. Only three things rally, the engine is sludged up and has low pressure, the main bearings are worn and therefor the pressure is low, I would think it would rattle pretty bad if this was the case, and third the oil pump is failing.