To really get the used motor oil clean you need to re fractionate(evaporate and condense) it. This leaves all the suspended metals and carbon and such behind. IMHO only then would it be fit to run through my engine. I had a friend that worked in a transmission shop that had a furnace that would burn used motor oil. I'm happy to leave it to them to dispose of waste motor oil.

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green 85 300SD 200K miles "Das Schlepper Frog" With a OM603 TBO360 turbo ( To be intercooled someday )( Kalifornistani emissons )
white 79 300SD 200K'ish miles "Farfegnugen" (RIP - cracked crank)
desert storm primer 63 T-bird "The Undead" (long term hibernation)

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