For what its worth...

I run our club's trainer. I run an OS .46FX on it, and fuel with 20% oil, all synthetic. The qyantity of Oil Goo + dust + grass clippings + chopped-up-bug-bits covering the plane and wing after a day of flying was... impressive.

To cut down on the amount of cleanup work necessary, I cut a length of 3/8" I.D. clear plastic water line long enough to run from the muffler to the tail of the plane. I rotated the muffler outlet so that it was pointing down and slightly at the fuselage. I then clamped the plastic tube to the muffler outlet with zipties. A few more ties were needed to secure the tube to the bottom of the fuselage.

This works brilliantly! 99% of the exhaust now dumps out below the fuselage at the tail - the only cleanup required at day's end involves wiping off the dust (and grass) (and dead bugs ). The engine also seemd to be a lot quieter, as the plastic tube soaks up a lot of the racket.