[watmough]

I believe that lead in petrol / gasoline still exists as there are not one but two effects from lead.

One is that lead helps promoting burning of the gasoline versus explosion or knocking.

Two is that lead is a great lubricant for the valve surfaces, preventing them eroding, leaking or sticking as they open / close. Eroding causes the engine to lose compression and efficiency, and sticking (as I have experienced first-hand) can cause a major engine failure by breaking the mechanism that opens/closes the valve.

[newnewpdro]

This is correct, but it's also been overstated how important the lubricating properties of leaded gasoline were in these engines - while the harmful health effects were understated. [1]

[1] www.walshcarlines.com/pdf/mechanicalimplications.d4e.pdf

[olivermarks]

I've found it varies by engine type. International Harvester light line SV engines for example definitely benefit from lead additives to fuel. Zinc additives in oil is another 'be kind to your engine' additive https://zddplus.com

[olivermarks]

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.