Has anyone experienced a black ring of soot or some other crap around the exterior of the exhaust tips. It is all around the tips, about an 1/8 inch wide. It wipes off easy enough, but I don't know if it's normal. I have a 2005, F-150, 5.4 L with Flow Master Series 40. I'm not using any oil between changes and my engine runs like a bear. It ain't sounding too bad either! What is this crap?!

 

Just soot from cold starts and just plain normal use. If the truck is running open loop when you first start driving it every day that will build it up fairly quick, but normal use also does it.