I have a '00 426 which means I take the header off to remove the oil filter (I love that design *grrr*). When I did this, I noted a buildup of black carbon gook in the exhaust port just prior to the header pipe. Is that normal or should it be cleaned? If so, how do you clean it. I didn't see any mention of this maintenance in my owners manual. It's an even film that's smooth, but it's built up to maybe 1mm in thickness (hard to say for sure). Again, any advice would be appreciated.