Based on subjective observation and since nobody can explain why the IO-390 develops sticky valves on Cyl 2 the majority of the time here goes. Even an expert like Mike Busch feels too cold of CHT's contribute to sticky valves. The only issue is cylinder 2 in the majority of cases is the hottest cylinder in an IO-390. This is my story and for now I'm sticking to it.
For the first 200 hours of operation, I ran 100W and after break-in 100W plus. (Florida mostly and Ohio summers) Around 25 hours my oil would become very dark and was taught to change on condition which I did. Sometimes less than 25 hours. Every oil change Blackstone would give me an oil analysis and typically around 2,500 ppm lead. Other numbers looked great. A friend suggested a multi-weight (20-50) and thought that might help. It certainly seemed to and now was getting ~ 40 hours at the same level of darkness. Blackstone confirmed still seeing ~ 2,500 ppm lead and thought all was good. After the third oil change time for a conditional and my borescope showed lots of black particles around my valves, some fairly sticky. Cylinder 2 was the worst of the 4. Another oil change and inspection and could see these particles getting worse. Not knowing what to think went back to 100W plus. Next oil change less particles. As of 500 hours the particles and gue have not returned. Was the 20-50 holding less particles in solution and that is why it was looking cleaner? Being a multi-weight did that help the blow-by and that is why it looked cleaner? I had 2 long cross countries and that did not seem to help the 100W plus oil get darker less quickly, thinking I was getting blow-by as the oil was warming up. It "seemed" the majority of the sticky valve 390's 20-50 was being used, or was 20-50 just being used the majority of the time in 390's? Again, this is all very subjective on my part.
