Why is petrol also called octane?
Octane is a specific molecule with 8 carbons normally found among the many different molecules in gasoline. Pure iso-octane (2,2,4-trimethyl pentane) was chosen as the reference fuel standard for resistance to preignition for gasoline fuels. So the octane number of a fuel is a rating of the preignition potential of that fuel compared to a fuel containing 100 %  iso-octane molecules. Since iso-octane is a mouthfull, the "iso" part was dropped and people just call it octane.  This is why some vendor calls his product petrol or gasoline or gas or octane or high test, etc...