I recently had an O-ring go bad inside one of the covers on the side of the transmission on the SDL. It took a while to figure out that the oil was coming from the cover. The O-ring less than $1.00, but it took two tries to get it in right. No leaks since.
The hard part is determining where the leak is actually coming from. I thought at first that the oil pan gasket was leaking and replaced it.
The leak did clean the outside of the tranny and I could see where it was coming from by tracing the clean area back to the cover. Yours may not be leaking enough to clean the dirt off the side of the transmission.