To assure that the oil only comes out the exhaust tube, you might add a touch of RTV(blue liquid gasket) in the bolt holes, the muffler half seam and where the engine connects to the block. It only takes a small amount to seal these joints/holes. I had oil seeping out these areas and as a result got more on my plane from the propwash. Getting the oil to just come out of the end of the tube cut down on the amount of oil that was deposited on my plane.

Check this thread out

I also rotated my back muffler half so the exhaust hole pointed downwards.

somegeek