Back in '76 I bought a Renault 5 which required premium fuel that over the next few years became unavailable as unleaded took over the premium pumps
100 avgas blended in worked very nicely, but gradually airfields began refusing to sell for automotive purposes because of legal pressures.
Tetraethyl lead is said to be a nasty substance; so octane boost is better from other chemicals.
That was the last French car I bought. It required a steady diet of wheel bearings and CV joints before it rusted out
After a move it turned out that my new landlady had an R5. She saved herself a bunch of money when she took my advice to get rid of it before 65K miles.