Wait, are you saying used motor oil actually has less of the nasty fumes than fresh stuff? Because that matches my experience too, I've quenched a few blades in both and the fresh stuff smoked way worse right out of the jug. The real problem I ran into was getting consistent results, the metal particles and sludge from the engine would settle in the bottom of my quench tank and then the next blade would come out with a weird patchy hardness. I started filtering it through a coffee filter and a old t-shirt before using it, and it helped a ton. Still got that burnt smell though, that never goes away no matter how much you filter it. It's a pain to deal with but when you're on a budget it works good enough for practice pieces.

Ha, yeah. I guess we're all just spinning our wheels trying to keep the eggs from cracking while the whole store burns down around us.

Man that's kinda like when I spent half a day chasing a short on a job last summer. Turned out a wire nut had a tiny bit of copper poking through the tape, barely visible. My boss just smiled and said welcome to the club. Frustrating but now I always double check those damn wire nuts before moving on to the big stuff. Funny how the little things humble you in this trade huh.