What's the story with O2 (Oxygen) sensors - two mechanics give different opinions

Here are the details, I have a 1993 Trooper S, manual tranny, with 118K on it. I have never replaced the O2 sensor, even when the light went on at 90K. Gas mileage seemed fine and no other lights came on.
It just went on for its 120K services and tune at the dealer (I just moved here and did not know a good shop). Prior to this service I had done everything the manual said regarding service (oil changes, tires, 60K and 90K services, etc). I did notice some decreased fuel efficiency at 110K and I asked one dealer mechanic if I should replace the O2 sensor now (thinking decreased fuel efficiency was sign a sign the O2 sensor finally was shot). His response was only if the O2 light AND the check engine light were on simultaneously - otherwise the O2 sensor is fine. Just for kicks, I asked another dealer mechanic 3 bays down from the first, and he said it was likely fine and that the decreased fuel efficiency was a sign that I needed a tune up and not a new O2 sensor. I opted for a 120K tune-up, in addition to the 120K service, and they said they would check and reset the O2 sensor instead (replacing it only if needed). When I picked it up the O2 light stills comes on and they did not mention checking the actual sensor (it was late and I was in a rush to get home). However, the car drives great and fuel efficiency *seems* better - but it is early to tell as I have only driven 50mi on it since I picked it up.
Which one of these mechanics is right - or is there a different answer altogether regarding O2 sensors. I am curious as I am getting ready to drive from the Midwest to Yellowstone for summer fieldwork and I would like to use as little gas as possible. I do have a couple of days if I need to get a new O2 sensor, but that's it.
Thanks in advance for your help ...
_____________________________________ If replying by email, remove the obvious junk....
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
02 sensors are cheap and easy to replace on your own, so why not get one just to be safe? I believe to turn off the light there is a switch behind the speedometer somewhere (at least on my 1991 is there is).

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
To reset the O2 light in your dash, remove the instrument cluster and unscrew a little screw that I think is labeled K1 or K2 and put it back in the adjacent hole, that's it, light is out for another 70K miles. I have a 128K on my 1993 Trooper LS and still haven't replaced the O2 sensor, We put it on the computer last week for an intermittent check engine light and I asked the mechanic if I should replace the O2 sensor, the computer said it was working fine so we didn't, The intermittent Check engine light turned out to be a faulty EGR sensor/switch. Hope this helps

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Motorsforum.com is a website by car enthusiasts for car enthusiasts. It is not affiliated with any of the car or spare part manufacturers or car dealers discussed here. All logos and trade names are the property of their respective owners.