I don't think I've heard of anyone that actually needed to change that seal.
I have heard from a LOT of people who were convinced that seal was leaking though.
Most, if not all of them found that the oil that had accumulated right below that seal was actually oil that had been thrown off of the rear chain as they were riding.
I suggest that you buy a can of Disk Brake Cleaner and give the whole area a thorough cleaning. While your at it, clean off the oil that dripped over the edge and has run onto the bottom of the oil sump too.
Then, take the bike for a long ride.
When you get back, examine the area closely.
If the oil is nice clean thin engine oil you seal probably is leaking.
If the oil is thick, black stuff it is chain lube.