Oil will normally leak from between the muffler pieces, especially if there's a small rough spot in the castings.

One problem with tightening the long bolt is that if it's too tight, it will break. The muffler will expand as the engine warms up and will stress the bolt. Eventually (a few flights), the head of the bolt will be snapped off and you'll lose the back half of the muffler. We tighten the long bolt just enough to keep the rear from turning and then use the lock nut to retain the long bolt.