After a few years, I pulled my O2 sensor to clean it (should really be done on a more regular basis). I took the "breather tube" - or whatever it's called that looks like a little dumbbell - in to the local dealer to make sure I got the right gaskets and see if there was an updated tube. I showed it to the parts guy and he looked at me funny... then asked if I had already cleaned it. I said "That's how it came out - no, haven't cleaned it". He looked at me funny again, as if he didn't believe me. Then he asked if I used RingFree. I said "all the time". He said "Well, that explains it. Most guys around here don't use it and that O2 sensor and tube get totally gunked up. We're usually cleaning them a couple times a year for every one".
That solidified it for me.