Here is my old oxygen sensor:




How do you think - is it its normal condition after several years, or it have been dead a years ago?
Tomorrow wil find out whether my gas mileage increased...

Well, maybe the Gear Heads will have more info, but it doesn't look blackened with soot (rich condition) and it doesn't look white (lean condition). It does look old and perhaps in need of replacement.

That'* my take on it at least.

Were you having an issue with reduced gas mileage?

Yes-yes-yes!
My first car (Chrysler Concorde 3.2l 2001) has about 17,5mpg (city) and this bonneville has about 10,5mpg (also in city).
I'd like at least 15-16mpg and 20 at most.

It'* fouled...I forget which type of fouling is that color.

You could read it with a spark plug fouling chart. The color would hold true.

The residue is due to an oxygen sensor that has been inoperative for a very long time, combined with a gasoline formulation that is high in Ethanol and lead contents.

I'm impressed!

Blow me away with experience and knowledge!?!

And I also was under the understanding that a steady diet of lead-rich fuel will kill an O2 in short order, followed by the cat.

Look at the location of car: Russian Federation, aka Russia. The gasoline and fuels are not refined and spec controlled like ours in the USA. Yes...that oxygen sensor will be short lived. That is the reason why many cars are spec for overseas operation witha different ECM/PCM chip and program, and the oxygen sensor operation is disabled for leaded fuel operation. One example is the GMC Syclones that were exported to the Middle East; check out the following link on how these trucks were modified for leaded fuel. The site shows the differences on the codes and functions; the sites are as follows:
http://www.syty.org/old/saudisy.html
http://www.syty.org/old/promguide.html

Ah. good stuff! Then, the bonneville most likely was not converted in any way and viola!, a more frequent need for O2 changes and possibly something to be gained by a cat delete.

Remove the converter and probably add an oxygen sensor simulator from Casper'* Electronics. I do not know how effective the simulator is.