The only true way to clean 20 years of gunk from an engine is to hot tank it. I doubt that there is a solvent in existance that will safely remove the sludge that builds up from decades of non-detergent oil, condensation and gas contamination.
In fact, breaking a lot of the sludge loose, chemically or mechanically, unless the oil drainback holes in the head are completely plugged, seems like an invitation for disaster. The caked-on sludge is inert...it's not clogging up anything so why go to any extraordinary measures if you have decent oil pressure. I can see pulling and scraping the pan and making sure the screen in the oil pump pickup is open if there is a significant lack of oil pressure, but going any further if you're getting oil to the rockers is to me, counter-productive.
Once you've rebuilt the engine and it's been tanked and completely cleaned, the problem will be eliminated. IMO...keeping a 23 year old engine running until rebuild time would best be accomplished without any aggressive internal degunking.
Dave
