sight or smell - gear oil is pretty rank. Before doing work on mine, I had motor oil seepage from rear main, valve covers and oil pan. I haven't done it, but there are fore/aft trans seals - you'll undoubtedly answer what oil it is once you pull the tranny. I damn near used an ice cream scoop to get the grime out of my bell housing this past summer. Fun times!
Check that DS turbo coolant hose for leaking while the trans is out (just replace it regardless - only time you can do it) that's at about the 11 o'clock position looking at the rear of the engine. I had that leaking, too.