good article Roger
If you look at figures 3 and 4, towards the end of the life of the canister, the blue and yellow are both good - any n butane is not good for cold weather, iso butane is much better
This is the same as my experience, isobutane is good down to maybe 20 F (or maybe 22 F).
I have been buying Snowpeak "Giga Power" at REI that is a propane/isobutane mixture that has worked good.
Actually, putting in any propane is unnecesary. It helps early in the life of a canister, but towards then end of the life of the canister it makes no difference.