Best thing you can do is clean up that side really good. That way you can easily follow the trail of oil to the source. As it sits, there are about 6 things on that side of the bike that can leak oil. Looking at the picture I can rule 2 out immediately, which is the shifter shaft seal and filter screen cap O-ring, as they are both below where that drop of oil has collected.
That leaves the countershaft seal (sprocket shaft) and the shift indicatr sensor O-ring under the sprocket cover, which is what appears to be the source of the oil from what I can see. It could also possibly be an O-ring on the inspection caps on the left case (silver cap over crank bolt and flywheel timing hole) or the O-ring for the starter reduction gear cover (round cover near the starter up top), but all three of those would have more obvious signs of leaking on the side of the case cover and most of it looks to be from the bottom or behind the sprocket cover.
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