I can't help you with the to seal or not to seal question, but I can tell you that the black stuff might not be silicon at all. To my understanding, there is a product made by loctite called 'loctite 5900' that MB uses to seal flange surfaces, and it resembles black silicon. It is safe for oil pasages, and gives a very good seal. It was discussed a time or two on the forum. I purchased some for about 25 dollars, and have found use for it here and there.