A few years back I met a guy who owned an oil change station. He was filtering the waste oil through a series of increasingly smaller micron media, then centrifuging the final oil to spin out the soot.

The resulting oil was honey colored and he burned it in an old VW Rabbit that was his parts car and experimental platform. His plan was to start burning it in his Ford F350 tow truck, but I never heard if he got there with the project.

The 'soot' that was collected in the centrifuge could be formed into stuff resembling the density of a hockey puck .