The fact that the oil pressure drops after the motoer & oil is warm is a strong indication that the main bearings may be worn. Rod bearing noise will increase when the vehicle is in gear and the clutch is allowed to slip putting a load on the motor.

It's my thought that other than lifter noise the motor should run quite.

Will the motor smoke after it has been running or warmed up. If so I bet a compression test will also tell a story.

I am willing to bet that a trip to the rehab clinic should be considered for you next stop.