I have read in some other posts that the sensor have an opening someplace that allows it to sample outside air for a reference. My understanding it could do it from the point where the wires go into the housing. It might "breathe" through the wires strands under the sheath.
I really don't know!

If the outside became soaked in oil it would have to be a whole bunch I am thinking.

310,000 is a lot of miles. How about your plug readings and the sensors vanes. They should tell you if its combustion fouling.

Is the engine running OK power wise?
This guy is in the cataylic converter way down under, right? The flow might not be right? Not getting hot enough on the side it mounted into?

A Lot of smut or gunk on it is a heads up. It should be a gray to whitish color or brown depending on additives in the gas you use.
You should be able to clean the sensor with a propane torch and read a signal with a voltmeter from it at the same time.

Since you have so many it would be worth a try to test them. Varing the heat and exposing the vanes to air varies the voltage. I use a spreading flame attachment on my torch.

It is very unusual to hear of such a high failure rate.

The answer lies within the sensor being contaminated or the converters flow is my best guess!

The code is leading in that direction. Maybe a new clean factory sensor can work fine while perfectly clean or lower flow rate for a little while?

I am definitely no expert in this field!

Phil