Leaky 904

As stated,more than likely a leaky gear shaft selector seal. Actually just had mine replaced.
There is a special tool to take it out from the top.Kind of hard with the tranny in the car. The other way is to take off the pan,take out the valve body,and replace it from there. This is how they did mine.If you are interested in a shift kit, this would be a good time to replace it. Maybe they will include it with the shift kit.