I use the Butane Propane mix in my K-27 and K-28. I've had no problems with tank failure, but I am careful to bleed off some gas after filling and before running so that no liquid is injected into the burners. I use straight Butane in my Shay, tram and Roundhouse Billy.
The mix I use all year round is Primus Power Gas 2202 in 450 g canisters. It is a seasonal mix of Butane, Iso Butane (highter pressure) and Propane. In the summer, the mix is 20% Propane and in the winter the mix is 30% Propane. So, it's best to buy it in the summer to get the lower pressure cannisters.
The best deal is to get a discount by ordering by the case through a local camping supply store. This way, you don't have to pay the shipping or Hazmat fees. The last case I bought about a month ago was $105 for a case of 12, or $8.75 each. The Primus cans have threads on the valve, so a screw-on adapter is needed to fill your tank. Primus sells a short spouted one, which are usually available where you buy the cannisters.