Deionized water is very aggressive, it search to recover lost ions and find them in the boiler metal. If in doubt, just add a few percentage of tap water (~3%) to neutralised it. It may slightly increase boiler scaling, but that is preferable to the weld destruction..
Some live steam locomotive manufacturers (Accucraft) tells that Butane shall exclusively be used, while most tall vertical bottle (Express, Rothenbeger) are mostly filled with a mixture of butane and propane.
What are the differences ?
Pure butane gas remains liquid below -0.5°C at ambient pressure. To recover it while cold, it is needed to heat.
Pure propane gas remains liquid below -42.1°C at ambient pressure.
As seen above, what is motivating supplier to add propane in their mixture is the ability to work in cold weather. It also shall not be forgotten that if the flow is important, the liquid gas vaporisation is cooling the whole bottle.
The problem is that stored gas (in liquid phase in the bottle) is heated, the pressure increase in the bottle. And the pressure in a propane bottle @ 30°C is higher than the pressure in a butane bottle @ 50°C. That makes a big difference for a live steam locomotive which generally ends up quite hot.
If some suppliers don't want you using propane or mixture, this is simply because they have designed their bottle at a limited pressure not foreseen for using propane in hot weather.
The mixtures are generally gases with propane part lower than butane part, which drive to intermediate characteristics. Those mixtures are generally done with 30 or 35% propane.
We can see in above table than at 43°C, the pressure of a mixture 30%propane/70%butane is twice as pure butane. That cannot be neglected.
I own an Accucraft Countess with a manual telling that it is safe to use gas mixture. Though, while opening the locomotive, I found a nice plate clearly stating: 'Butane only'. Gas connections supplied by Accucraft for the filling are built for bottles which could only be found with gas mixtures (30 or 35% propane). A minimum of coherency from this supplier is required.
In doubt, it is preferable to comply with manufacturer instructions. If you wish to operate in winter with butane, store your locomotive and bottle in a well heated room before use and everything shall be OK.