I heard back from the "Ethyl Corporation", although not much help. I'll paste their reply and keep on researching.
Mr. Freudenberg -
I am the art curator for NewMarket Corporation (Ethyl) and your request for information was sent to me. I have checked some old ads from "The Saturday Evening Post" and "Life" magazines which is the only place I could think of that I have noticed the old Ethyl trademark. Since most of the ads are cartoons, not photos, I have no way of telling whether they are the round porcelain signs you are referring to. If you feel confident that your round sign came from an actual gas pump, there were tons of cartoon type photos of a round Ethyl signs on gas pumps. The oldest one was from 1930. My guess is that the signs on the pumps were metal not porcelain, but I don't know this for sure. Not sure where the 8" round porcelain signs would have hung.
Sorry that I can't be of more assistance.
Diane M. Hazelwood
NewMarket Services Corporation
(804)788-5522 - Phone