phat
As I suspect you know, I own both the Coleman Powermax stoves, the Xtreme and the Xpedition (doubleburner) and they use liquid feed of propane butane mix and I've used mine at -5 F and it worked great. I'm pretty sure it would still work at -20, but you'd have to add some heat reflected back on the fuel bottle to work much colder. White gas or kerosene are the preferred cold weather stoves but I hate the smell of kerosene burning so I'd use white gas in my old MSR XGK.