Wait, you've been running the same plate for years without ever changing it? That blows my mind considering how much the texture changes just going from a coarse to a medium grind.

Same here, nobody told me for years either.

The stuff that made your hooks look so clean is likely the carbon and fine metal bits floating in that old oil, they can act like a mild abrasive and help knock the flaky scale loose while the oil cools the steel. That said, the smooth grey finish you got probably has more to do with the quench speed than the dirt itself. Used motor oil is actually a bit thinner than fresh oil because of fuel dilution and breakdown, so it cools faster, and faster cooling is what pops the scale off. Dale's 10W-30 being nasty is a happy accident, not some secret trick. Where it bites you is on bigger pieces, because dirty oil can cool unevenly and you can end up with soft spots or cracks you don't see until you snap the blade. For hooks you're gonna beat on, run the dirty stuff all day, but for a knife that matters, spend the few bucks on clean oil and you'll sleep better.