Sometimes a sludge and/or varnish buildup is all that's keeping a worn out engine working- - -particularly if a bunch of carbon has built up behind the piston rings. I've seen a few old engines turn into mosquito foggers after an aggressive crankcase cleaning- - - -the gunk behind the rings went away and they quit sealing compression and blow-by.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!