Extending Spark Plug Life
Wow!....$11 per spark plug ($132 for a set for my ML-320). I think there is a way to save some bucks. The working part of the spark plugs are good for 100,000 miles with quality unleaded fules and modern ignition systems. The reason the plugs should be changed after four years has nothing to do with the plug performance, but is to prevent corrosion from taking hold that could damage the heads if the plugs are removed after many years. Changing the plugs every four years breaks any corrosive bond that is forming before it gets too strong.
In a few months My ML-320 will be four years old and I will remove the spark plugs; wire brush the threads to remove any corrosion; inspect the gap; lightly coat the plug threads with a high temperature Anti-Seize compound, and re-install them. Since the Anti-Seize compound is also a lubricant, I will torque the plugs to the lower torque limit ( I think about 20N-m, or 15ft-lbs)
If I see any indication that the plugs are not in excellent condition I will install new ones, but this is not likely.