Hello Daniel. Sorry to say that used motor oil does make a good quenchant. Very inconsistent and not good for you. If you do not want to spend the money for an engineered quench oil, canola oil will work, however it does have a shelf life and degrades with repeated quenches.

Brion

When we quench, our goal is to EXTRACT HEAT at a rate that the steels makes its specific transformation into the  phase of steel we seek. Dedicated quenching oils have specific additives that assist in this heat extraction - completely unlike motor oil.

Motor oil's additives are there to aide lubrication.

The only thing they have in common is that they're both liquid.

Old time Bladesmiths and Blacksmiths, loved old motor oil, but those were simpler times when we didn't know what we know now; we also worked with asbestos without a second thought.  Even good quench oils are not really good for you to breath, but motor oil is a toxic soup of who know what.  But beyond this I could show you micrographs of the inside of blades that really says it all to the old timers who said that motor oil worked "just fine."  They were able to say this because they weren't aware of the results on the inside that we can now see.