Why do you feel the need for a flush? I'd say with the mileage you have, if the previous owner changed oil as recommended, sludge would not be a problem. What causes sludge are short trips where the engine never warms up to burn off water and acids that accumulate in the oil. Time is as important on oil changes just like miles. My concern would be if there is some grit in there thats not hurting anything. If it were to come loose and the filter didn't get it, it would end up in the clutch plates. I've read where car restorers will empty the oil, put in some cheap oil, run the motor a few minutes, then change with the good stuff. This will help clean the inside.