My '89 300E with 116k mi just had its head rebuilt because of a cracked head gasket and a warped head. I can't recall how much was milled off the head but my trustworthy mechanic tells me it's minimal compared to other MBs he has seen.

After putting about 300 mi on the car, I checked the plugs and found that all 6 have a little bit of black carbon build-up (like a coat of gloss black paint) and the plug threads are soaked in black oil. The car runs like a champ and doesn't seem to burn oil, not enough to notice anyway. Also, I have records of oil changes every 3k miles since the car was new from the previous owner.

Is this carbon build up normal? What does this say about the condition of the engine. Thanks in advance.