WAY BAD CALL 94JIMMY!!!
A finely tuned motor does NOT even use the O2 sensor. A finely tuned motor uses all of the O2 which gets burned in the combustion process, and the O2 sensor does no work. A poorly tuned motor lets O2 get through the combustion process (by not burning it all), and into the exhaust, and the O2 sensor tells the ECM that compensation is necessary to fix another problem. If you'd like to see that in print, I'll be happy to get it to you. A bad O2 sensor is an extremely common myth!!!