if its leaking by the trans, that is the driver side, clean it up and rtv the pan up, that way you will know if its the rear seal.
i also had a guy tell me my rear seal was leaking, and my power steering pump as well. it was the valve cover, so take what they say with a grain of salt. i had just seen my seal with my own 2 eyes 3,000 miles before when i had the trans out, and it was dry. ( i had the leak then too)
if you take off the inspection cover from the trans, by the starter there, 2 bolts, you can see if its leaking, you need a good flash light and eyes. the flex plate would most likely will have oil on it, and the the back of the engine as well. yes it is tight, but you will see something for sure. thats the area i used a fat screw driver to smear the rtv over the pan gasket area.
white Shepard, i never heard of. i'd weary about adding anything to your oil.