A process for recovering used lubricating oils, and oils recovered using the process. In a first embodiment (for used industrial oils), the used lubricating oil is mixed with clay in a reactor. The mixture is preferably heated to between 80 and 200 degrees Celsius. The temperature should not be too great, to avoid “cracking” the oil (i.e., breaking molecular chains in the oil). After a certain period of time, the mixture is pumped through filters. Cakes of clay and contaminants remain in the filters, while the oil emerges without the contaminants. A second embodiment (for removing ash or soot, very fine carbon particles and other organic compounds from used motor oils) is the same as the first embodiment, except that before the mixture is passed through the filters, a centrifuge is used to remove most of the clay contaminated with soot, so that it will not block the filters.