I am trying to figure out why my oxygen sensor failed ?

It was a used unit for the junk yard. lasted me about 2 months (the new one I had lasted about 2 years). So maybe it was already old to begin with. But what bothers me is that i have a small oil leak under my car. Consequently, the whole underside of my car has a thin layer of oil. I guess I get this is from the spraying/wind effect as the car is moving. I also see this oil/dirt on the oxygen sensor wires. After some reading up on these sensors, I wonder if these oil contaminants are shortening the sensor's life ( apparently the way the sensor is constructed, air can get in). I have got a new one on order, but I would like to know why it failed.

Any comments? Are sensors from junkyard very prone to failure within short periods of time? Did anyone have problems with these sensors due to contaminants?

Thanks in advance.