I once had a guy come in at 60,000 miles for his first oil change in a Toyota 4Runner. He insisted that we put an oil cleaner in the engine to clear sludge that we found in the intake manifold and oil filter.
That sludge was sealing the piston rings and once the oil cleaner cleared out the sludge the car left leaving a trail of blue smoke a quarter-mile long.
I'm fairly sure they tried to clean as much of it as possible just with the valve cover off. I think at that point you have to say, we need to completely tear down this motor and clean it from top to bottom. I mean it may be some more down-time, but I think that it would be more beneficial than running some mystery oil through the motor for a couple of thousand miles and flushing it out.