HT Issue in Metallurgy and other enigmas Posted August 15, 2007 I would also use a fast oil - one that is meant for quenching - there are a lot of them. ATF fluid is mineral oil (usually) and it would be too slow. Motor oil is an unknown, and likely to stain the parts. It also has an unknown flash temperature. There is a reason quench oils have been developed, to get properties and properly control distortion. There a variety of sources, even some on this board that sell them in the necessary quantities.