A petrol (gas) question

[Stichill]

Something interesting, tinboatcapt, that I remember from a research paper I wrote on the detrimental effects of low-lead gasoline on engines designed to run with leaded fuel, is that the octane-boosting properties of TEL are not linear. There is a relatively large octane boost for the initial amounts of TEL, but further octane increases require ever-higher concentrations of TEL. So that seems to fit your impression that a small amount of leaded avgas would have a larger-than-expected effect on the "virgin" unleaded fuel in the tank.