I mean it'* either working or not right?

Not necessarily. It can cause you to run lean or rich (usually rich) without totally failing.

My mechanic friend said its good to change it with every oil change

If you're still getting over 25mpg Highway Driving, than I would leave it alone. I replaced mine at 80,000 miles just because I was going on a long vacation and it was piece of mind. I only had one O2 sensor, but I have read(on this site) that some have 2(I think it'* the OBD II series). Obviously, if the SES comes on and gives you a code that points to the O2, you know it'* time to replace it.

o2 sensors are WAY to spendy to change with every oil change. Maybe once a year max, but 3 or 4...c'mon :?

You need a new mechanic. O2 sensors typically last 50-60k miles before exibiting signsn of weakness or failure. There are premature failures, but changing a $50 part every oil change is the stupidest thing I've ever heard. Are you sure you heard him correctly?

Yea, mine costs $90...And I asked this question becuase i debating on weather or not to take it back. I'd rather have the $90 than unchanged fuel economy and performance.

Hmmm... brutal.

Scammer... lol... oh well.
*about paying for a new o2 senser ever oil change

im getting scamed?