the oil is from the engines intake pulses, a small amount of mix will slightly blow out of the carb when the engine is running just the nature of these engines, over time the fuel will vapor off but the oil doesn't
so from time to time you will have to wipe it off or clean the air breather I don't oil the filter as it will get oily on its own when I wash out the filter with dish soap and warm water. Once its washed I wring it out mount it and go for a ride dries it out pretty quickly.
A little spray of brake cleaner on the carbs outside and the engine will help keep the engine clean.
If you use carb cleaner on the outside of the engine when its mounted on the bike you might ruin your paint carb cleaner is rough on paint,skin, lungs,and very bad in your eyes. Brake cleaner is almost as bad. Depending on what the cleaner are made from some can be very nasty.