Power steering fluid instead of ATF?

Just don't use it for frying chicken.

All I can contribute about quenching oils is this: The real ones are formulated to avoid bubbles on the workpiece when you plunge it in. Bubbles mean local soft spots, stress, and even possible cracking. Don't use used oil for that reason; it often contains entrained water or acid, and that's hell on your workpiece.

Or, buy the real thing. But it's bubbles that are your enemy.

The way I understand it, oil is slower to boil than water so it quickens the time the part quenches. PS fluid is a bit thinner than most oils, so it would be a slightly slower-hardening liquid, but pert much same-same. Still quite a bit different from water, so the O-2 hardening should be "safe" and proper.

No, oil is much slower, and it has nothing to do with boiling. It's slower because its rate of thermal conductivity is 1/4 that of water, and water has more than twice the specific heat of mineral oils.

Water-hardening steels require a very fast quench to harden. Oil-hardening steels require a medium-speed quench. Air-hardening steels will harden at the slowest quench rates.

Viscosity is not it, either. But oil is a lot "safer" than water. Water quenches oil-hardening steels too fast for safety, although thick sections of O1 or O2 may require water to harden at all. And then they may crack. For safety in hardening thick steel, you want the slowest quench-rate steel.