Bosch says O2 life for best performance is 30,000 miles max.
Beyond that the O2 degrades and functions slower.

You cannot see inside the O2 tip as that is where the sensor is so the shell color could look good ( esp when w/meth useage reduces carbon buildup) but still be dirty internally.

Your country might as they do here in the USA in spring time switches fual makeup.
Here they add more ethanol as much as 10 to 15% which reduces fuel mileage as it does not produce the same power as straight gas.