I would advise pulling at least one valve cover and see what it looks like. If it's just has a coffee colored film over everything, that is better than if it has a heavy caked deposits filling most of the valve cover, (usually from only adding oil and never changing it). If really heavily caked, it probably would be best to accept it needs to be torn down and overhauled to give reliable service.
If not too caked, in years past working on old motors I have had success with two methods, one by draining a quart of oil and adding a quart of ATF, or the second method by draining two quarts of oil and adding two quarts of kerosene. In both cases run the motor in the driveway for at least half an hour to fully warm it up, periodically revving the motor to 3k rpm or so. Then drain the oil while hot, add new oil and filter, drive street speed for a couple hours, and drain hot, replace oil and filter.