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Sorry, but you cannot use different brands of fuel according to FlipFuel, they tell you that you should only transfer the same brand to the same brand, so MSR to MSR, but you can't transfer MSR to JetBoil. Not that it won't work, it's mixing two different fuels could cause a problem. I don't know enough about this stuff to tell you why a person can't mix two different fuels, but you could probably e-mail FlipFuel and ask them why you can't mix.
Well, yes, I did reach out to FlipFuel and they "do not recommend" mixing fuel brands.
On the other hand, their advertisements and videos prominently featured on their site all show their
product in use mixing different brands of fuel.
I was unable to get a straight answer as to
why they do not recommend mixing different brands of fuel. My own suspicion is that the problem is that even identically sized canisters from different brands have different empty and full weights, and it would be fairly complicated to be 100 percent sure you are not overfilling a canister -- which is a
very bad idea. At best overfilled canisters perform poorly and waste a lot of fuel. At worst they might explode, although I very much doubt that if used as instructed you could ever get a canister overfilled enough to explode. Although it would be a fun experiment cooling the receiving canister with liquid nitrogen and heating the emitting canister in very hot (not quite boiling) water. But like I said if you are cooling the receiving canister in the freezer and heating the emitting canister in the sun you probably can't get to major overpressure.
I've seen this gadget in use in field conditions (again mixing fuel brands) where they cooled the receiving canister in a snowbank and heated the emitting canister in very hot (but not boiling) water in a pan. Seemed to work just fine.
Given that all of the major brands of canister fuel use a mixture of propane and butane (sometimes isobutane) I can't imagine there is a risk of some kind of chemical reaction between the two compounds. Somewhat more possible is that since the propane is in solution with the butane, different butane isomers might be able to "hold" more propane, so if you mix fuel types you will "lose" propane and might have a weird kind of overpressure situation. But that should be mitigated by the fact that if you are just mixing partial canisters, the propane inevitably gets burned off first (higher vapor pressure) and it is hard to imagine (that word again) how you'd end up with too much propane in a recharged canister.
So I'm going to be (carefully) mixing different brands of fuel and I'll see how it goes. If something
really bad happens I am sure to make the news so keep your eyes peeled.