I was told once by someone who had a car with a large V8 in it, that when working on a Devon squadron, any 100LL collected when legitimately draining the tanks on the Devons found it's way into his car... diluted down, the car ran without any issues at all.
My father' cousin also ran his old Austin on 100LL when serving at St Eval in the 50s... by all accounts it had sparks coming out of the exhaust, so not sure how long the engine lasted on that one!