I had this problem on my cr125. The first thing you should do is order in an exhaust flange gasket. This is a metal gasket that connects the pipe holder to the cylinder. Those gaskets seem to go farely often.
The other thing that you need to do is jet your bike. Do you have any experience with carburetor jetting? Next time you go riding go into an open place and do a full throttle run in all the gears. Then when you top out full in the clutch and hit the kill switch. Coast to a stop and take a look at your spark plug. It should be a mocha tanish color. If it is black you are rich (which in your case you will be rich because of the black oil coming out of the silencer) If you are rich on your main jet order a main 1 and 2 sizes smaller and do the same until it is the correct color.
Now adjust the pilot. Set your idle to a farely high idle and turn the air screw in and out for the highest idle. If this highest idle is above 2.5 turns out you need to go to a smaller pilot. Keep doing this until it is in between 1.5 and 2.5 turns out.
The other thing that you can do is raise the clip on the needle. This goes all by feel. You will feel the difference when it is not running rich. Good luck, and if you have any questions you can email me as well at firstname.lastname@example.org
How much came out, if the answer is a lot then i had the same problem with a 250 RM. A main engine bearing colapst. This broke the oil seal to the crank. The engine oil was sucked into the cylder and spat out the exsaust. The bike still ran even after all that.
is it spooging from the silencer ? it sounds like you might have ajetting problem. what jets are you running? how far out is the throttle screw? what ration of premix are you running? how long since last rebuild?