If you are running the motor and oil starts seeping out from between the mating surface, you need a gasket. Just use some silicon RTV...make sure that the mating surfaces are clean and oil free. Put the RTV on the muffler mating surface and turn the muffler mounting bolts till you feel the slightest resistance or you see RTV starting to ooze from the gap. Then let the RTV "set up" for a couple of hours. Once it is set up tighten the mounting bolts...by letting the RTV set up first it will keep it from oozing out and prevent it from getting in your motor.