Burning used oil in a furnace still sends all the heavy metal contaminants into the air. Don't worry, not preaching, I burn it in my garage furnace!

I burned used motor oil in my last car, as well as WVO, as well as ATF, or a combination of all the above, as well as anything else I could find for free, anything to not have to burn diesel! Although the car was a junker and only drove for a year before scrapping, never had ill effects from the various cocktails floating around the fuel tank. I wouldn't do it to my SD or recomend it, just reporting. (the fumes from my exhaust would burn your eyes)

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1982 300sd, new project car to restore

It doesn't matter if you know what you're doing, as long as you look like you know what you're doing.