There was a time when plugs weren't changed. When I first got into the trade we had a plug cleaning station at the GM dealership I worked in. By that time, it was rarely being used since selling new plugs was more profitable. If you look at a plug, there's not much to go wrong so long as the insulator and electrodes are ok. We cleaned, did a visual check, gapped them, then reinstalled and the vehicle ran like new. I used to use it to clean the plugs on my older snowmobiles that were often oil fouled. Today, those cleaning machines are rare collector items and are highly sought after.
I've got close to 50,000 kms on my plugs and still haven't changed them. New ones are ready to go but they look good, gas mileage is still ok and engine runs fine. I might change them in the spring to see if I can notice any performance change.
