When understanding the principles of cannister stoves and winter fuel mixes, it's good to start with a simplified analogy. Imagine three pots on your stove. Each pot has 1l of water. You start A at a rapid boil, B goes at a low boil, and C is just simmering. You let them go until A has about 100ml's of water left and 900ml's has evaporated., and then shut them all down. On observation, you notice that B perhaps has something like 400ml's left, and C has about 750 ml's left (actual amounts would vary, this is for illustrative purposes only). Now think of those pots as A=Propane, B=Isobutane, and C=N-Butane. If those three were mixed in a cannister, when the cannister is upright and feeding a stove, they boil off in a similar manner to the above analogy. So when the cannister is partially full, it now only contains a fraction of the original propane, and the majority of the remaining fuel is butane. Butane boils at -0.5 C, isobutane at -12C, and propane boils at -42 C. If the outside ambient air temp is merely -8C, at some point long before your cannister is empty, the flame turns orange, and then dies (even if the cannister contains 100% isobutane). Why is that? Because all that evaporation means the cannister is significantly cooling (propane is actually a highly efficient refrigerant, as people in third world countries have been well aware for years). When a cannister is upright, it is feeding the stove with gas vapour(s).
The solution to this dilemma is to change to liquid feed. When a cannister feeds liquid to the stove, the ratio of the fuel mix stays constant to the bitter end. To feed liquid to a stove, the liquid fuel must be preheated so it will vapourize before reaching the burner. So in order to use a cannister inverted, you must use a stove that has preheat tubes. Trying to invert a stove that does not have a preheat tube is dangerous. In order to optimize their fuel mixtures, many avid winter outdoors people have resorted to obtaining their own adapters creating their own propane/butane mixes. Everyone that uses a Cricket lighter knows Isobutane is a far superior gas to refill their lighters with than n-butane. In fact you can distinguish a weekend warrior from a professional by watching what they light their stove with. The former will often produce a BIC from their pocket, whereas the latter will have a variety of implements, none of which will be a BIC lighter. But as a fuel for cannisters, Roger Caffin has found that the performance of an inverted stove using isobutane is no better than a stove that uses a straight propane/n-butane mix ("...pure isobutane canisters, which seem to be dearer, may not be really worth the money: you might be better off using a standard 70% butane/30% propane canister and keeping it warm."). As butane has way less pressure than propane, butane cannisters are much lighter and thinner walled. Commercial manufacturers of butane cannisters have to plan for the worst case scenario of someone leaving a cannister in direct sunlight in the window of their car, so they have to be very judicious about the amount of propane they put into a winter mix. As reported in the Backpacking Light forums, experienced mountaineers however, taking this fact into account, have taken to creating their own custom seasonal mixes. Take for example someone that creates a 40/60 mix in their unheated garage in November using the proper equipment and procedures, stores it there, and takes it on a trip when it is -30C outside, and then purges any unused cannisters of this mix before summer rolls around (at which point in the heat of summer he will only be using a cannister that is 100% n-butane that he fills from the Asian Market fuel cannisters) has a better understanding of risk management than casual recreationalists*.
* Manufacturers of both butane and 1lb propane cannisters state that these bottles are not refillable. While there are a number of enthusiasts who undertake to do it anyway, doing so improperly can result in injury or death, and can be illegal in some jurisdictions.