At that temperature range, you're going to have to rely on "tricks" to get them to work. Warming the canister in your coat helps alot. Another trick is to set the canister in dish filled with water when you go to use the stove (helps keep it from cooling off when you use it in freezing weather).

As for the fuel mix, you don't want anything but isobutane and propane. Im pretty sure that MSR has the most propane compared to the rest (there might be another one out there that's better, but it isn't in any of the stores around here so I can't remember).