Tetra-ethyl lead was produced by the Ethyl Corporation specifically as an octane booster, beginning sometime in the 1920's. Valve seat cushioning was simply an added side effect of the lead salts left over from the burning of the octane booster.

http://www.britannica.com/science/tetraethyl-lead

People who were yet unborn or filling their diapers when lead disappeared from automotive fuel probably shouldn't offer opinions about why it was there.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
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