ALL O2 sensors 'drift' over time. They degrade slowly but predictably; high temps and contaminates [Silicon, Phosphorous, etc.] in the exhaust are the main culprits.

Winter gas is oxygenated to further reduce CO emissions. It has lower BTU content per gallon - that is why your mpg drops, you need more gallons for the same energy to drive X miles.

Hot rodded 2 cycle snowmobile and boat engines used to burn pistons from overlean A/F before folks learned to deal with the extra O2 in reformulated gasolines.

So - it is very possible that an 'old' O2 sensor with a lean drift bias coupled with oxygenated fuel could make your engine run overlean - extra heat from lean mixture may contribute to the pinging.

Check your spark plugs for color and condition - they would be the best indicator of sustained lean/hot operation.