Oil sight glass change

[His Dudeness]

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.