Engines of previous eras were designed to leverage the 'lead' in gasoline as a lubricant and act as a 'cushion' to the valve seats and valves as the fuel flowed through the engine. Not adding a 'lead' substitute can damage your valves, and ethanol is to be avoided at all costs to protect the engine.

Engine 'knock' (multiple explosions on a single piston ignite cycle as different pockets of fuel mixture ignite) is also minimized by having 'lead' in the fuel.