Although the plugs are good for 100,000 miles of life, I thought Mercedes recommends a replacement at 5 years or 100,000 miles (whichever comes first). I know of some owners of 1998 MB products that are just doing their first spark plug change after 6 years with no problems of plug seizure which I though was the reason for replacement by time. Do the plugs really need to be replaced every 5 years or is this a waste of time/money?