Hello kfox -

I have always wondered if recycled petroleum products such as used engine, hydraulic, and transmission oils - etc... could be used to produce diesel fuel. I don't think that the process of making biodiesel (transesterification) can be used to make diesel with these petroleum products. If I am not mistaken, to get these petroleum based oils to a diesel type of oil would propably require craking them in a refinery. Although mixing them with a solvent might solve some of the issues of getting the oil to run in a typical diesel, but may not be good for some of the newer more sophisticated common rail diesels comming to market.