No, you don't have to drop the tranny to replace the oil pan. You do have to drop the exhaust though. Replacing it is not a 1 hour affair, it's kind of a pain.
There is usually a difference in smell. GL products have a fairly unique smell to them as well as being much thicker. Easiest way I have found is to get some on your clothes and walk by the wife. If its motor oil she is quiet. If gear lube I get kicked out of the house almost immediately.
To further the question of rear main seal. If it is leaking and needs replacement is it an OEM part or is there another preferred brand to use or stay away from. Thanks.