i think autogas has a mix of propane and butane
This is true for pretty much all propane. Technically, the oil industry calls it LPG - Liquified Petroleum Gas - in order to maintain a bit of ambiguity as to what exactly is in there. The percentage of actual propane in LPG can vary by quite a bit in different countries, but in the US it's generally around 90%. There is also iso-butane, butane, butadiene, butylene, propylene, methane, traces of other light distilates, and of course the ethyl mercaptain that is intentionally added as an odorant so you can smell it when there's a leak. This site has a great explanation:
http://www.propane101.com/propanegradesandquality.htm
