I checked my owners manual and it indicates that spark plugs should be replaced at 100,000 miles or "5 years." Gilly had previously indicated a 4 year replacement.

Based upon what Gilly is saying, plug replacement must also be based upon a time interval due to the possibility of the sparkplug seizing to the head. I always thought that was the reason for using antiseize compound when installing a sparkplug.

My 2000 ML320 has only 37,500 miles on it so far and would sure like to get more miles out of my 100,000 mile sparkplugs than they are going to have in 5 years of use. Seems like a total waste of 100,000 mile plugs to me.