Chemistry of things like fuel have benefits and penalties in achieving a target such as a "octane rating"
They are not tabled as such as they are by products of a requirement of the fuel.
Changing any part of the chemistry can keep the same "octane rating" and other performance items like say burn speed.
But can have massive changes on other things due to chemistry change - say more acidic for example, that brings on corrosion in humid conditions on valve faces that accelerates wear.
A single atom can change things dramatically.
Lead seems to have effects on cooling and lubrication in certain applications as a by product.