Recommendation: Keep the reciept when you re-fill.. Butane is less expensive than propane and selling it as Propane is FALSE ADVERTISING.
Others have explained that Butane has a vapor temp of around the freezing point of water where as Propane (Rounded) is forty below (your choice of scale).
NOTE: in neither case is that the exact temp. but it is close enough for this post.
Another issue.. IF it is a portable tank with outside threads (ACME) connectors then the trailer hose has to fit on quite tight, There is a valve that must be depressed or propane does not flow.
The reason for this...
There is a plastic washer in the hose end fitting, if it melts the part that goes into the valve will move out a little bit (Melts due to say a fire) shutting off the gas flow.