Factory change interval is 15000 miles on the factory Champion RC12ECC plugs. At that mileage the plug gap is opened up and they do show wear. They need to be changed. I have been using NGK ZFR5N V power plugs, at 15000 miles the gap has opened, but they are showing almost no wear. I have 2 sets of them like this. I am thinking of just cleaning them, re-gapping, and using them again for another 15000 miles. Has anyone re used these NGK plugs after re-gapping them?