I got to tell you ATF will smaoke to beat hell and every once in a while a ball of fire will roll (roaring;) up through the smoke! Don't stand in the smoke with polyester clothes on. ;) Real quenching oil hardly smokes at all, has additives for that and to increase the speed and modifying the cooling curve etc too, plus the additives were chosen to keep the good aspects of it going as long as posible. Quenching oil that stops working right has to be changed out so it's been a competition to make some that holds up well and does a good job. Getting the "real thing" for a hobbist just makes it so you only have to buy it once and eliminate quenching problems at the same time. I should have said- "a big target is important for long-thin water hardening parts". Some steels need less than a second to quench to -catch- the "austenite" (orange-hot soft stuff) and bring it down to below 1000F in one second (for 1080) so that it will decompose into "martensite" (extra hard brittle stuff) instead of decomposing to "pearlite" (medium-hard tough stuff) like what railroad rail and the top-half of a cold chisel is made of. That information is from "isothermal transformation diagrams". The "point" on the curved line of the graph that you've got to get around is call the "knee". The knee for O1 tool steel is at "1100F and 7 seconds" and is an oil hardening steel.
5160 (it's like tool steel type-220 or a certain kind of L2) I don't have a IT graph of it. Never got around to buying the book that has it. A 3/4" thick rod of 1080 and oil quenched won't work for full martenite transformation, not even the surface. For oil quenching 1080 etc they have to be thin for oil to get full hardness. But full hardness may not be what you want in a sword. :) The 3/4" 1080 rod's surface will be 15% pearlite and 85% martenisite and the core will be 100% pearlite. Fig. 5-16 ASM's Tool Steels by Roberts and Cary