There is a sleeve that slides down over the wire(s) connecting the sensor
to the electrical system.   The sleeve seems to be made of some type of
heat-resistant but permeable material.
 I understand, perhaps mistakenly, that this sleeve is there to allow
ambient atmosphere to get into the top of the O2 sensor and somehow the
heated element in the exhaust stream compares this ambient air and sends a
calibrated signal to the engine controls...yes?  The 'sleeve' where the
wires emerge through the outer end of the O2 sensor must be to keep stuff
from plugging the ambient air access?

  Here is what I am asking for....I recently replaced a faulty O2 sensor in
my inline VW (Cabriolet motor) powered Vanagon.   I also have an
increasingly leaking axle seal on the same side as the exhaust system, just
ahead of where my O2 sensor is mounted...(I know, fix the dang leak!...but
I am trying to wait a few more weeks before starting my fall van R & R
chores till when the weather goes bad here in the Northwest)

   So, the axle shaft throws significant oil all over that oxygen
sensor...That sleeve is saturated with gear oil and the whole area is an
oily mess.  But for a week or two since replacing my O2 sensor the van has
been running better than ever, until this last tank of fuel.   Now my gas
consumption went back to appx where it was without an O2 sensor...and the
running temperature has dropped, to the point where it runs with the O2
sensor unplugged...Rich mixture again, I think.   I checked all the
connections and theylook fine.
   So I am thinking that having oil all over the O2 sensor and saturating
that sleeve, that could be causing me problems with the O2 sensor ...Am I
on the wrong track with this supposition?   Could my O2 sensor be bad again
so soon?  (Bosch single wire about two weeks old)
   I don't want to drive around at 17mpg when I normally get 23-24.   If I
have to, I'll waste a nice Indian Summer day and fix the dang seal on the
output to the left axle...but I'd rather hold off a bit longer if I can..
  anyone?  thanks  Don Hanson