Check to see if you all the bolts holding the crankcases together are tight. If they are and its still leaking oil, then you should probably take the engine out of the bike and take the cases apart. Then replace the the gaskets and while your at it all the main crank bearings and seals. But if you dont want to do all this then you can take some epoxy and put some on the seam. This should stop the leak, if it doesnt, then you'll have to take the cases apart and do what I said before.
Also, if its a tiny tiny amount thats leaking than I dont think that it would be a problem, just as long as only a tiny tiny amount of oil leaks. But definitaly try the epoxy thing even if its a tiny amount.
Best of luck.
Jim