Maybe it had thicker oil in it at the start and it now has thiner oil that would increase clearances when hot. if high detergent oil is used in old motors it scrubs the old gunk out of the motor and plugs the suction screen in the oil pan and oil filter, causing low oil pressure.
if the old oil was like tar and the new is like water the oil pump crank and all the bearings could be worn. you need to get an oil pressure gauge on it and see whats going on. or put thicker non detergent oil in and run it to see if it cures it.
The worst option is theres low oil pressure which means the motors up for rebuild and to be honest a tractor of that age with poor previouse owner maintanence could be needing it.
Thats the best I can tell you without seeing the tractor and doing some test work on it. Regards Robert