Gearcase oil is a good thing to service anyway, if it hasn't been done for a while might be a good idea. If gearcase oil is leaking out, it will likely have water leaking in too. It will look like really milky if there is water in it. Most of the time I see an oil slick behind my rig, it is fuel or oil leaking down from the powerhead somewhere. I've replaced a lot of two stroke fuel lines and hose clamps over the years after seeing similar symptoms.