I am resurrecting this thread as @Peter pointed to it from here
And I was asking where this comes from
Peter wrote:
Probably because of low cylinder pressure for extended periods, causing the piston rings to not seal too well, and with oil leaking past, you get glazing.
Now I want to derive something from that information. What would be the prerequisites to run an (air-cooled) engine in a normal to low power output regime? If there’s a comparatively low oil consumption would this mean that there should be few oil deposit (sludge) in the cylinders?
I don’t burn (or loose) a lot of oil. And I don’t find amounts of sludge, albeit I don’t run my engine on high power outputs. Rather around 50% and loong cruise flights. However, I fly on quite low RPM which in turn should increase cylinder pressures.
Now one could think that a low oil consumption is a prerequisite if you want to run an engine on lower cruise power, because otherwise it gets full of sludge. Is that thinking correct? What do you think?
I think those who actually know are not on forums.
My guess is that general oil consumption is not related to whether one glazes the cylinder bores with extended periods of low power flight.
Of course after they get glazed you will have a high oil consumption 
As far as I could gather from other forums (…), from experts such as Mike Busch and others, from different publications, and from my own experience:
I agree with all except possibly #3:
extended periods of low power flights do not lead to glazing if you do the occassional normal flight
I can’t quantify it but perhaps 30hrs, all at high altitude (say FL170), non turbo, can bung up the rings on my IO540-C4.
In practice, in GA, this very rarely happens.
And this is where it all gets complicated…
There are sooo many configurations, engine brands, oils, ops Ts, flying or operating styles, that any conclusive evidence remains as an isolated experience.
E.g. I’ve now flown my baby for > 1’200hrs in 4 1/2 years, most of this time at between 40 to 50% power settings, and guess what? The oil consumption is exactly the meager same 1qt/50hr as it was when I started enjoying her (…), and the cylinders, which get borescoped every 100hr show no less no more glazing either.
Steering back to the title of the thread, suspecting sludge build-up associated with an increased oil consumption would mandate the flushing procedure in the cylinder(s) concerned.
What is true about RPM: if I set low RPM isn’t cylinder pressure higher, so low power and low RPM is fine?
Hello Europe
Cylinder glazing is as mentioned in one thread, happens as a result of improper breakin. I would say its impossible to have it happen in service, no matter what oil or power setting you may use.
Hi consumption will be a result of glazing at break in. In later hours, there are more factors, including insufficient choke or wear due to improper installations and in TCM systems, fuel scheduling.
As far as the “procedure” as mentioned, the only person that has talked about it is Mike B. I can pretty much guarantee. that he has never personally signed a log book using that procedure. Lyc has SI’s that counter that process. There is NO way to get all the contaminants out of the case and cylinders after you fill a cylinder expecting to loosen and free up stuck rings. This is something farm boys do. Myself and other IA’s here in the states are shocked at the idea of such a process.
As far as running engines at 50%… if the poh and lyc or tcm have charts for that low power settings, you should be fine…. Key is being able to have oil TEMP in the good range to burn off contaminates. I would suggest more frequent oil changes and perhaps use of straight weight oils as added precautions for longevity. CArl
Thank you @planewrench for that professional view. I very much appreciate that!
planewrench wrote:
As far as running engines at 50%… if the poh and lyc or tcm have charts for that low power settings
Yes it has, and yes I monitor CHT and oil temps closely. Interestingly enough (at least for me 
) my engine seems to have an oil cooler bypass valve, which seems to be the reason why my engine keeps the oil temp quite well.
That is what I was thinking right from the start. When the temperatures are fine, noise and vibration are fine, there should be in principle no limitation to run an engine in any power setting. That in mind: too low power settings that lead to potential damage in the long run will be indentified by too low oil temps. I assume that about 180 °F could be a lower limit for “good temp”.
What is this sludge made up of?
My answer to everything is lean aggressively, unleaded Avgas and Aeroshell 15W50.
Peter if this is your aircraft why not fill not tank with UL91 and use that for the cruise?
Because practically nobody is selling UL91
