what is the moisture or humidity like in your area, especially at night? POP CAN THEORY, When the temperature outside is hotter than that inside ( a sweatting soda can ) in-other-words, when you shut the car off after heating the exhaust system sufficiantly, the moisture from the outside air collects all through out the exhaust system. Mean while, it collects soot that is all through out the pipping, and deposits it in the form of spray and or a constant dripping. It wont smell like fuel or oil, its thicker than water and thinner than oil, and the dark color comes from the pipe itself. roughly meaning, its not some thing to freak out about. the motor will collect condensation through out the coolling proccess (be it minutes or hours) and naturally spit them out every time you start and run the car. I have a black spot on my garage door ( about 2 feet in diameter) that is from my 1970, 120,000 hard miles, 240Z. RELAX, but have it check out just for your own peace of mind. If i'm correct, me and Beandip both live in the N.W. of the united states and we're used to it. Dave.