I decided to replace my plugs myself after the stealership quoted me $120. Followed the instructions from another post (thanks josh) and didn't have any issues replacing. However, I have a question regarding the appearance of the old plugs. I have a V6 w/ 33K. I had 3 Densos and 3 NGKs. All plugs had turned a dark brown at the base of the insulator. Almost looked burnt. Densos were worse than NGKs. I compared my plugs to the ones in josh's post and none of his have this discoloring. So my question is, is this normal? I've done some towing but I always moderate my speed. Any insight would be appreciated.