Typically it would be noted as a drop in fuel economy without any other noticeable drivability symptoms. They are essentially a battery that is capable of generating 1 volt. A chemical reaction happens when the inside of the sensor is vented to the atmosphere and the outside of the sensor is exposed to the exhaust stream. The higher the oxygen content in the exhaust... the more voltage it will produce. But being a battery they wear out. Typically north of 100K on an O2 sensor it will start to lose it generating potential. This issue is much easier to see on vehicles with OBDII. You can actually monitor O2 voltage. OBDI system can't do this. They're also prone to failing with the use of some of the older silicone sealants. Most of them these days are O2 sensor safe. But if the wrong type was used... it coats the outer layer of the O2 sensor thimble and renders them inoperative. Most time they will set codes to when failing but not always.
If this is the same symptom you've been dealing with for months... I kinda doubt it's an O2 issue. But... if it's the original it certainly can't hurt to install a new one.
Unc, this is the weirdest 2.9 ever in my 87. After replacing the tank, both pumps/filters, new dist. w/TPS, MAP sensor, wires, plugs, timing, it was noticeably better. After running a couple cans of Seafoam through a few tanks of gas it seemed to be getting even better. Checked a fuel injector and the mechanic said it looked fine. However I am only getting around 7-8mpg. ????? It idles and revs nicely. Under load it struggles and on a hill I can't get past 1800 rpm in 3rd gear. So, I'm thinking 02 sensor and injectors. Can't hurt right? only the pocketbook.