i use non foaming hydrolic oil. but my quench tank is a old drive shaft about 18" long so it is a small volume of oil. if you use this method, fill tube till about 4-5" from the top that way you can heat your oil and thermal expantion won't make a mess,but i have used motor oil,tranny fluids, i mainly use what i have enough of to change the whole amount out,avois thick oils,hope this helps, jimmy