Thanks for the good info. You actually answered another question I was going to submit regarding black oil in my Volvo Penta TMD22A (5000 hours) and my Onan KMDB 6500 Watt genset (3000 hours). Until recently, the oil in the Volvo would remain clean until changed again, but after a recent change (I change oil and filter every 150 hours) it immediately turned black. I attributed this to perhaps sludge breaking loose in the oil pan. On the other hand, the oil in the genset was always black even after an oil and filter change (I change oil and filter every 100 hours). I attributed this to incomplete drainage of the oil which is removed by gravity drain. As in the Volvo I assumed that there might be sludge in the oil pan.
Your solution to add an oil cleaner and run for 6-8 hours is one I had considered, but recalled when I did that on my 1951 ford flathead in 1960, it removed the sludge but also caused the piston rod bearings to loosen up and resulted in a failed engine. Since I only paid $10 for the car and used it for two years, I wasn't concerned and just bought a new $10 car (51 Chevy). Unfortunately, a $250,000 boat is a different story than a $10 car.
Considering both the Volvo and the Onan, do you know if adding the engine oil cleaner additive you suggest would clean out the sludge but also loosen up the bearings given I've never done it before? Since you apparently do this on a regular basis, that hasn't been the case for you. My concern would be that since I haven't done this on a regular basis, I might have a different result. I'm currently in Europe, but will consult with a specialist when I return to Fort Lauderdale where my boat is located for Hurricane season.
S/V Pacific Cool (Amel Super Maramu Number 141)