Membrane separation of used oil and compositions generated

A process to re-refine used oil, comprising:

• introducing a used oil comprising soot content and metal content, and a solvent to a separation unit under separation conditions selected to produce a purified oil product, wherein the separation unit comprises a porous membrane, a semiporous membrane, or both;
• and
• separating, via the separation unit, the used oil to obtain a diffusate and a retentate, the diffusate comprising the purified oil product and a portion of the soot content making up 0.09% or less of the diffusate, and the retentate comprising a remaining portion of the soot content not in the diffusate and at least 90% of the metal content, wherein the metal content of the retentate isolates the remaining portion of the soot content not in the diffusate away from the diffusate,
• wherein the porous membrane or semiporous membrane has a molecular weight cut-off of 2,000 Daltons or less, and
• wherein the solvent is selected from the group consisting of pentane, hexane, heptane, benzene, toluene, xylene, methanol, ethanol, 1-propanol, methyl ethyl ketone, methyl isobutyl ketone, and combinations thereof.