Richard Riley wrote:

Back in the days when unleaded premium was new, there was some folk
wisdom that you could mix leaded 92 and unleaded 92 and get about 94
octane out of it - that the efficacy of the TEL as a booster decreased
exponentially as the percentage went up.

It's news to me. However, there are many misconceptions about what the
octane rating really is, which can lead to many theories about power from
engines. That almost sounds like one of those urban legends.

Many people think that premium fuel has more energy than lower grades,
when actually it is about the same. What the higher octane rating
represents is the anti-knock properties of the fuel, and a high rating
typically means that the fuel burns more slowly than fuel with a lower
rating, and is less inclined to preignition. There is no extra energy in
the fuel.

Therefore, if a car engine designed for lower octane fuel is run on
higher octane fuel with no modification, then the mileage will be exactly
the same, ignoring the energy effect of any additives. The only way to
get better mileage is to increase the compression ratio and advance the
timing, which increases overall engine efficiency. You need high octane
fuel to do that.

A jump from 92 to 94 octane is huge. It implies that the TEL is boosting
the no-lead portion of the fuel by 4 points of octane, which is more than
the effect it has on base gasoline. I don't know enough about the effect
of lead to say it's impossible, but it certainly seems a stretch.