Going off the original post, and the question. You've stated that the motor has lost oil pressure since when you first got it and now. That's not normal, period. You've either got an oil pump that is failing, or bearings that are failing, or both. I personally wouldn't be comfortable with the situation, and would seriously consider pulling the motor and checking your bearing clearances. Buying a used motor, I would have plasti-gauged #5 and #7 rod journals at a minimum, and the main bearings before installing.