Not too long ago spark plugs were replaced every 30,000 miles. Then 60k became standard and now it's 100k. 30 years ago spark plugs fouled from deposition. New engines run cleaner, which means wear has become the limiting factor.
The spark plug manufacturers went to exotic metals to reduce wear, and for the most part those materials work. However, wear of the carbon steel electrodes is still an issue, and a tired engine can actually foul the exotic metal spark plugs more easily than before, because the exotic metal contact area is smaller.
These images clearly illustrate wear in a modern high performance spark plug, from a car that just arrived for service. The top photo shows how the electrode wore at an angle, and the top-on view of the plug shows how the deposition pattern (left side only) reflects this pattern.
As the plug wears in a sloped pattern the flame front becomes more and more one-sided which leads to knock and misfires. The worse it gets the more this particular cylinder's timing will be backed off to compensate, with lower efficiency, more heating, and less power as a result. Uneven plug fouling will raise the necessary firing voltage, which will over stress coil packs.
This particular plug came from a 2009 Range Rover with 80,000 miles. It's clearly due for replacement even with 20% life remaining by the schedule. The moral of this story - manufacturer service schedules are a guide, not an absolute. Some cars will benefit from earlier plug changes.
Remember - as a carmaker their goal is to sell new cars, and part of that plan rides on wearing out the old car while another part rests on low advertised cost of ownership (defer it all till the last possible moment!)
As an owner, your goals may be rather different, especially if you plan to keep your car a long time or pass it on to a friend or family member.
John Elder Robison is the general manager of J E Robison Service Company, independent restoration and Bosch Authorized Car Service specialists in Springfield, Massachusetts. John is a longtime technical consultant to the Land Rover, Porsche, and Rolls Royce Owner's Clubs, and he’s owned and restored many of these fine vehicles. Find him online at www.robisonservice.com or in the real world at 413-785-1665