Ross, have you done anything with the tractor such as changed the oil in it? The tractor could have gasoline dissolved in the oil from an improper carb setting ,which would thin the oil and give you a low reading. I would suggest an oil change with a 30-40 hd oil. Sometimes an additive like STP will boost the oil pressure. Old M's sometimes show a drop in oil pressure when they warm up and thats more normal than abnormal.Check that first. Next I would look at removing the pan on the M and checking the pick-up screen for sludge and blockage in the pan. It's likely a good idea to clean the pan out anyway. After this check you may need to go to the oil pump itself. Our M will drop oil pressure as it gets warm, so does one of my H's ,clearly visible drop on the guage. Good luck from Mike in Exeter Ontario