Gilly:

Sorry, but I am just a stubborn old german who has a hard time believing all of these "extended" maintenance schedules that Mercedes (and other manufacturers) promote to make it appear that operation costs for their vehicles are low. I am one of those diehards who believe in maintenance if you expect anything you own to last.

You have convinced me to replace my sparkplugs at both a time/mileage interval. However, if the time interval should be 4 years, Mercedes should be picking this up under their maintenance warranty. Like you say, sparkplugs are 2 bucks apiece (less at dealer cost) plus whatever labor time is allowed for this procedure. If Mercedes is comfortable with a 5 year interval I will probably go with that since my 2000 ML320 has only 37,800 miles with most all miles being highway miles.

It seems as though all manufactureres are going to 100,000 mile sparkplugs, but until this thread, I have never heard of replacing sparkplugs on a time interval for those of us who do not put a lot of miles on our vehicles each year. It makes sense to me and it appears that there needs to be some consumer awareness done for all owners of vehicles equipped with these new 100,000 mile plugs.

P.S. - This may not be an issue for some of those American vehicles out there where a 100,000 mile sparkplug may be a "lifetime" plug. It may even outlast the rest of the vehicle. HeHeHe!