There's no such thing as a clean plane when using smoke oil. Some end up not as wet as others because the owners use a better smoke fluid and they adjust the volume of oil entering the muffler. Tramsmission fluid would not be high on my list of smoke oil ingredients if I was making my own in any case. Usually less oil equals more smoke, but you will end up with an oily plane no matter what you use. You can't put a boat in the water and not get it wet....

You might want to change the title of the thread. You can't blame the muffler for putting oil on the plane. You did that when you elected to use smoke oil