I remember from organic chemistry over sixty years ago, that
it was thought that a straight chain hydrocarbon with 8 carbons, would be the ideal fuel. Gasoline in early days was a mixture of more than a few length chains. In addition to straight chains, hydrocarbon chains can have forks, and
some of the the bonds can be double, unsaturated. It finally turned out that the very best single type had a fork, and I think at least one double bond. But I can't remember the details. I'll see if I can find it.