Agree, there's something wrong but with any luck it might be something very simple. If there's black oil going down the pressure line you should be seeing it out the muffler as well. The usual cause as Jim suggested is if the muffler isn't tight enough on the engine and any tiny movement will give a lot of black from the aluminium being fretted off. But it's also possible (assuming it's not a new engine) that the muffler has old caked on carbon inside it that's gradually getting washed out.