The 100K miles or 5 years should have stated "which ever comes first". The factory does not put any anti-seize on the plugs or dielectric grease on the boots. You will find not only the plugs difficult to get out but the plug boots will be a pain to remove as well. Plan on spending several hours to replace your plugs.

My 98 E320 is on it's third set of plugs at 163K miles. I changed them out every 80K. The factory plugs were the most difficult to remove, since then I treated the plugs and boots and removal has been much easier.

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Ray
1998 Mercedes E320, 200K Miles
2001 Acura 3.2TL, 178K Miles
1992 Chevy Astro, 205K Miles