I don't have my handbook of Chem and Physics here, so I can't put together the formula for boiling point depression, but I was able to verify Craig Connally's numbers in "The Mountaineering Handbook" that butane boils at -11C at 3000 m(10,000 feet) and at -19C at 7,000 m.

I have used my Pocket Rocket w/ MSR fuel down to 0C or so, but at 10,000 feet. The 123 worked OK down to -15C or so, but I had a pump. Below that I have always used my XGK.

Respectfully,

Jerry