Leaded fuel won't really hurt anything mechanical in the engine (the extra lube with the lead is like added protection) - but anything which gets exposed to the exhaust can get a coating of residue which can impact the effectiveness of that sensor (like an O2 sensor or the insides of the cat converter). Again, my history on the "killing" of a cat converter was in the days when they used "beads" inside the converter. I'm not really up on the latest converter technology, and whether this lead will impact it long term. I'd imagine a tank with lead wouldn't hurt in and of itself, but longer-term use might... Of course, longer-term environmental impacts are there given the whole issue with Lead in the first place, but that's not part of this discussino..
You're probably OK, but it might explain the residue. If you switch stations to a place that's known not to add things to or dilute the gas, and the problem goes away, then you've got your answer.....
