Had a long chat with Peter Feachey of Energshift, they did the conversion on our Disco back in 2005 before we bought it, and he is going to fit the additional gas tank for us later on this year.
He says:
Autogas is basically Propane, generally only the Europeans mix the two gases for when the weather is cold, but we run in the UK on Propane.
Propane by itself is fine and is also cleaner, Butane is a bit 'dirty' by comparison.
Using the big 47kg red propane bottles is fine, but for the hassle of getting the gas out, putting up with the debris at the bottom of the cylinders and having to pump the stuff out, he reckons it just isn't worth the time and hassle.
Peter