[A4] drifting gas mileage display

[Peter Kirby]

I am looking at buying a 2008 A4 Avant.  During our test drive last night,
I noticed that on a flat road with steady throttle, the instantaneous fuel
mileage indicator would slowly drift up and then slowly drift down, etc.
Is this an indicator of O2 sensor going south?  I can't get a straight
answer out of the owner about oil consumption but am wondering if oil
consumption is contaminating the O2 sensor....  Any thoughts.

Thanks.

Peter