Just to correct an error, the sight glass is listed as a spare part 4H7-15361-00, You could always drop the sump as you should be able to clean it from there. If it is that bad it might be worth cleaning the sump pan and pick up screen while you at it..
I'd guess it's mostly from used oil sitting in the engine for a long period. The older the oil is (in terms of mileage) the more combustion contaminants it has in it. And the older it is the more the additives are used up. There's additives for keeping the contaminants suspended in the oil so if they're used up and you have a lot of contaminants from the oil being old when you park the bike the contaminants are going to be released from the oil settle on the inside surface of the engine. That would be my guess.
Don't forget though, when i'm not riding, the bike is on the side stand, so oil doesn't even touch the glass.