Buying "winter-grade" bar oil is the easiest by far... no muss, no fuss, no stink.
Last year I was using some stuff from the fleet store, marketed by Chain Pro ($4-5), that was supposed to be good in any temperature. The stuff was kind'a impressive, stayed about the same (fluid) thickness warm and sub-zero, but when I stopped last week for resupply it wasn't on the shelf anymore. I really had to swallow my pride and buy some Husqvarna "winter-grade" ($7)... it was a fight, but I finally got my Stihl to accept the stuff without spitting it back in my face. Stihl makes (or markets) some really good "winter-grade" bar oil, and I like it a lot... but at over $10.oo a gallon it ain't that good!
If you need to thin your bar oil the use of kerosene, diesel or fuel oil (same stuff, different label) is preferred because they are "oils" and also have a lower flash point than say gas, mineral spirits and the like. Light weight oils like hydro and 10w will work, but they're thicker than kerosene so it takes more of it to do the job. You'll be surprised how much a half cup of kerosene will thin a gallon of oil. But really, when you're already "there"... well, necessity tends to make a fella' creative... 'cause I ain't drivin' back to town for a half-cup of diesel!
Hmmmmm, seems like I have some Marvel Mystery Oil in the tool box? Nope, no Mystery Oil... Wonder how this gas line anti-freeze will work?