My dad owns a Ford 4610 diesel engine (3 cyl.) powered tractor that he bought used approx. 20 yrs. ago. Condition at time of purchase was very good overall, in my estimation. It was mfg. approx. 1990 time frame. Yearly use since he has owned it is around 40 to 50 hours per year. Since he bought it almost 20 yrs. ago, the oil has only been changed 4 or 5 times by me. NAPA oil filter of the correct type/part number used each time, and Shell Rotella 15w-40 conventional oil used each change. The last time I changed the oil/filter, the filter was very heavy weight wise, and was full of very dark gray/black sludge. I had not really noticed this before, but as I said the oil has been changed very few times since my dad bought the tractor 20 yrs. or so ago. The oil that was drained from oil pan was very black, but flowed smoothly into the drain pan. Temperature that day was around 70 deg. F (engine had not been run prior to oil change). The engine uses very little oil and does not smoke excessively. Also, engine coolant leaks do not seem to be an issue, as far as coolant leaking into the engine oil system. Does anyone have any experience with the Ford 4610 series tractors concerning oil changes that could provide input on this matter ?