Is the oil in that area clear and thin or is it thick and black and gummy?
If the oil seal is leaking the engine oil will be very thin and (except for the dirt it's collected) it will be fairly clear.
If the oil in the area is thick and black, you are seeing chain oil that has been thrown off of the chain during your riding.
That flat ledge right below the drive sprocket always collects chain oil which will eventually run down the crankcase fin to the bottom of the engine.
Left uncleaned for a long period of time it will eventually drip on the floor.
Use something that cuts oil along with a few paper towels to clean the area off and then keep an eye on it.
Like I mentioned, if you see thin, relatively clean oil, the seal is leaking and the dealer should fix it.
If it's what I think it is and it's chain oil, just wipe it clean every so often.