I've nevered worried about a muffler leaking oil. The oil gets everywhere with a 2 cycle engine, so some of it coming out muffler fittings instead of just blowing out the exhaust pipe is of zero concern to me. As the guys above posted, use some rtv or gasket material if it really bothers you. The main concern is that the muffler stays tight and secure, and it doesn't fall off the plane.