Sorry guys I have to disagree. Lead was discovered as an octane booster in the early 1920s by Charles Kettering under the direction of GM as an employee of Delco (which he owned and sold to GM). The sole purpose of his work was to find a cheap and effective octane booster, because those 20 hp engines in the 1910s had 6ish:1 compression. Oil comes out of the ground with an octane of about 60. The lubrication benefits are secondary (they actually had to add another additive to keep the lead from building up/sticking to everything), lead is an incredibly effective octane booster, and also helps keep engines cool as well as lubricate the valve seats.

I did quite a bit of research on the history of this a few years back, it was absolutely put into use as an octane booster. Excerpt below from his Charles Kettering's Wikipedia page and sums it up well.

"Kettering's research in fuel was based on his belief that oil would be in short supply and additives would allow more efficient engines with higher compression. His "high percentage" solution was to mix ethanol with gasoline, while his "low percentage solution" looked for additives that would be added in small quantities to increase what later would be called the octane rating of gasoline.[18] Thomas Midgley, Jr. and Kettering identified tetraethyllead (TEL) in December 1921 as an additive that would eliminate engine knocking at a dilution of one thousand to one.[18] While use of ethanol could not be patented, TEL's use as an additive could. Kettering and Midgley secured its patent and proceeded to promote the use of TEL as an additive instead of other options.[19] Kettering became the first president of the newly founded Ethyl Corporation that started to produce TEL in 1923.[20]"