Here's one you will not believe, I didn't until I saw it with my own eyes.
I flying buddy of mine had a couple engines that were covered inside and out with burnt oil, tried a dozen different ways to clean them with poor results.
Then a mechanic friend of his said...and I still can't believe it, take an old crock pot, NOT YOU WIFES GOOD ONE! add antifreeze and boil the engines disassembled for a day.
They looked NEW! I still think its witchcraft, but I am going to have him slow cook a couple of my old near and dear engines.
Hope this can help someone.