There have been times when our repair shop has found spark plugs seized in cylinder heads of vehicles with as little as 60,000 miles on them. Removal of these seized plugs can result in hours of labor. There are documented repairs where it was necessary to remove cylinder heads just to get the remove the plugs. We suggest that to maintain peak performance your spark plugs should not be run longer than 30,000 miles. Car buyers should take those 100,000 mile spark plug marketing claims of manufacturers with a grain of salt.