It occured to me recently that my 13-year old 148000-mile 325iX is
running with its original O2 sensor. No complaints from the ECU and
out of sight, out of mind applies. I see that the recommended
replacement interval for this thing is 60K. Oops. That said, if it's
working, is it really necessary to replace it? If so, is that special
tool BMW recommends (it looks like a cross between a crow's-foot
socket and a shark's mouth) really necessary? With it seeming to be
working correctly this strikes me as letting a sleeping dog lie. Or
am I giving up power/performance/mileage that's not immediately
apparent? No problem with emissions testing, BTW.

If "replace it" is the answer, who do you like for the replacement
part? What's a good price?