Well lets say about leaning out in it most aggressive state. Now this aggressive to the max but proves beyond doubt what happens . As ive stated before as you lean you slower the burn rate is. If you really lean you have the chance to have the burn on overlap the wrong way, That is now the intake is open and the charge ignites the in the manifold. Don't believe me well I suggest that you take a short trip to your local drag strip and watch blowers being lifted of.
Now lead actually acts a lub on the valve seat. Its a seconder action its primary use is raise its octane rating. Low leaded car engines require harden valve seats to stop wear on the seats and valve. Also something from my memory bank that radial engines are different by memory they also use 130 octane fuel that was around back then cant quite remember the exact figures though im sure someone knows it.
The cost of putting additives in any product cost $$$$$ in these days if they don't need them they don't use them.
Work beckons now go see what your science shows me today pmsl