So far I've used canola oil heated to 110F for quenching 5160, with excellent results. Someone suggested I quench in used diesel oil. His theory is this: The black soot in the diesel is "free" carbon that is not chemically bonded with the oil molecule, and therefore a quenched blade will absorb some of this free carbon yeilding a slightly higher surface hardness, as compared to a blade quenched in clean oil. Sounds plasuible. I've done a small test piece. My super accurate Rockwell-testing file reads "Yup, that's pretty darn near full hardness, buddy". Does anyone else quench in used motor oil, and any opinions on the "case hardening" effect supposedly due to free carbon in the quench?