CS,

The O2 sensor tells the ECU (brain box) what the state of the fuel mixture is from the oxygen levels in the exhaust. When the sensor fails the mixture can go rich. O2 sensors can last for 100K or more but can also fail sooner if contaminated by things like anti-freeze or silicone from souces like RTV silicone sealers. There is alot more about this is the BB archives if you care to research it. Also items like a bad coolant temperature sensor or fuel pressure regulator frequently cause overly rich running conditions. Glad you got the problem resolved. I would change the oil and filter right away if a significant amount of gas has gotten into the oil. Gasoline contamination/dilution compromises the oil's ability to do it's job lubricating the engine. I'd recommend a Mann filter and 10w30 Mobile 1 synthetic oil.