[redshift]

(metallurgy) Animal, vegetable, or mineral oil, such as fish oil, cottonseed oil, or lard, used in quenching baths for carbon and alloy steels; removes heat from the steel more slowly and uniformly than water.

As long as it has a high flashpoint, I don't know how much it matters. I think normally it's between 40 and 90 weight standard oil. But they do sell dedicated products if you want to be particular.

It is critical that you shove the part into the oil in one quick motion, or else you may end up with one side harder than the other.

Looking great man!