O2 sensor breakin?

[Mike Del Tergo]

I just swapped out my O2 sensor after having a bout of poor mileage (23 100% 
highway) and pulling codes (confirmed O2 probalem).  I had replaced it 5 
months back with a general tune up, but after I replaced all vacuum lines 
and other gremlins, I assume I fouled it.  IN went the new and my mileage 
barely went up .5 MPG, again all highway.  Is there a breakin period for 
O2's?  My first tank or 2 with the car I did get 27+.  I also get 27 on my 
other avant wth twice the total miles.  No mods etc, same gas.  Oil smells 
more like gas than I would like and oil consumption was up a bit.  I assume 
I was/am running somewhat rich and cutting the oil a bit with too much gas,  
Any BTDT's welcome.
Mike

_________________________________________________________________
Set yourself up for fun at home!  Get tips on home entertainment equipment, 
video game reviews, and more here.   
http://special.msn.com/home/homeent.armx