Some more holiday curiosity here in australia many vehicles are powered by LPG which is typically a propane butane mixture and it is quite cheap at about $0.50 per litre. On the other hand it costs about $1.50 per litre to fill a bbq gas cylinder with propane gas. It is in fact illegal to fill a bbq cylinder with autogas, and people are advised it is quite dangerous to do so, but there seems to be no consistant reason given for this. Many believe it results in more CO2 produced during burning. Others say it will be a hotter flame that the bbq was not designed for. I have even heard an explanation that the propane and butane liquids will separate in the cyclinder which of course is rubbish.
One thing that would obviously occur is that taking vapour off the cylinder of mixed propane and butane must lead to the liquid being enriched in the higher boiling component ie butane as the cylinder MTs. but why would that matter? surely either butane or propane will cook my steak quite adequately? ( I know the vapour pressure curves for both gases and that pure butane will have no pressure in the cylinder when the temp gets down to about 0 deg C, but thats pretty rare in this part of the woods, and if its that cold I aint bbqing no how!
I am no expert but I assume in vehichle use the car takes the liquid lpg from the cyclinder so the composition wont be changing with time. Terry