Hmm, I wonder what's happening there? Lead obviously does this but that's a new one. Could it be gloop present in the engine that is being cleaned out by the detergents, i.e. the actual source of the stuff is in fact oil?
Don't get me wrong, I can live without tears and misery if my car is belching out hard sulphurous pollution but if it is going to eat my hego sensors and cost money, that's a *quite* different issue.
The other car ran leaded and seems to run a lot better on higher octane fuel. I'm slightly relieved it can run on unleaded at all, truth be told, rather than VSRing into oblivion.