Hi, this is my first post so bear with me. I have oil on the left side of the drive pulley. There does not seem to be any oil behind the pulley, I did not pull the pulley  off yet.  I will attempt to post 2 pictures. I did not see any posts about a leak in this area.
I did have the drive belt too thight for a while[1 year orso], until I read the info on belt tension on this site. I hope I did not mess up the seal that I assume is behind the pulley.
Thanks, Gerrit.

Release the two bolts that hold the starter and wiggle the starter out. If there is little to no resistance, the starter o-ring is bad and leaking oil and dumping it through the drain hole right into the area you describe. With a good o-ring the starter should snap in very tight.

That oring has been known to fall into a hard to get to place. I've never pulled a starter, I just think I remember hearing about that happening to guys. Be careful,

So I pulled the starter, came out fairly hard and the o-ring looks good.
No oil residue around the drain hole at all.
I think my 2 pics will show, the oil is just on the outside of the pulley, I don't see any oil behind the pulley, although I did not pull it off yet[din't want to poke the bear ]. The oil seems to have started around the nut and spun out when I took a short drive to warm it up and change the oil.
I can't see how I accidentally could have spilled oil on that pulley.
Do you think I should pull the pulley off?
I also got the clutch cover off to inspect the chain tensioner [18.5mm] and the head cover off for the leaky plug. As long as I can tell where everything goes!

Well maybe you need to poke the bear.
And you need to check if the pulley is working itself back forth on the splines, which will only get worse with time.
To get the nut off, I used to put the front wheel against a wall, put it in gear, stand on the brake, cuss and even then the tires would just slip.
But I found that if I put a ratchet strap around the outside of the rear tire to the frame and just sit on it to get enough traction to get it off.

Go ahead and poke that bear.  As a minimum you need to verify that the pulley is good & tight.  You might as well simply take the thing off and see what's goin on.

I'm sure you are aware that there is a lip seal back there.  You need to check it out.  There's also an o-ring inside the spacer behind the pulley.  That o-ring sits in a groove on the inside diameter of the spacer.

To loosen the pulley nut, you run the bar through the rear pulley and so that it is situated on top of the swing arm. Like this.

You want one of the THICK pulley webs/spokes (green checkmark) to hit the bar, not one of the thin ones (yellow X).