When I bought my '97 Wrangler TJ,2.5 4 banger, it had 92,000 miles. Oil was black, but no sludge. I changed the oil and filter and checked the oil after about 200 miles and it was black. I changed the oil and filter after 2,500 miles, check it at about 150 miles and again the oil was black, still no sludge. I have changed it the 3rd time and the oil turned black after a couple hundred miles. The engine shows good oil pressure, it does not use any oil and there are no leaks. The new plugs I installed when I bought the TJ show no fouling with about 7,000 miles on them. What could be causing my problem and should I risk adding a cleaning agent?