I've got an 88 rm250 and I would say its also a bit on the spoogy side. The bike runs great so I don't suspect any mechanical problems. What gas/oil ratio are you running? I run 32:1 (now changing to 42:1) and I too get some gook coming out. But on these old bikes, I don't know that it signifies anything significantly wrong. You could try changing out the exhaust gasket on the cylinder, and double check that your exhaust pipe springs are good and tight as well. Overall, how does the bike run? The transmission oil is not the likely source. You would have to have a very blown crank seal and a very well worn piston and ring to have it coming out the exhaust. The bike would not be running well at all. The bad air filter will not cause this either. If the bike is truly "over spooging" then its either because its jetted too rich, or your premix ratio has too much oil in it. Its a 20 year old two stroke, they're not known for being overtly clean and tidy. Its funny how I got attached to mine though......
Trev