This has been something that appears time and again on a Monster centric board I'm on. 99.9% of the time it's just condensed water vapor. Of course extra easy to point at that on the air cooled bikes.
Cool weather and short rides where the bike does not get up to temp for a significant period promote this milky appearance.
I'll recycle a little write up I did there - Here goes:
What the hell is that milky crap in my oil sight glass?! OMG is my bike going to blow up?
- Don't freak, you'll be fine (well you hope). That milky stuff is just moisture. Lots of short rides where the engine never really gets above 100 deg C can promote the appearance of this milky stuff. Lots of wet weather, cooler temps etc will also bring it on.
If you have an air cooled Monster, your worries are over as it's not a sign of anything bad happening with your bike. If you want to get rid of it, simply ride you bike until it's nice and hot, say 30min+. Then when you stop for gas/lunch/nose pick simply remove your oil filler plug for 30 seconds give or take. You'll probably see water vapor escape from the open filler port. Do this a few times and the milk should disappear.
IF you have a water cooled Monster follow the steps above - but also keep a close watch on the coolant levels in your over flow tank. Make sure to do any measuring under that same conditions - like bikes cold and parked the same way in the same spot, to prevent mis reads because of fluid expansion, tilt of the bike etc. If you coolant level is mysteriously going down, and you have a persisting milk issue, that's a good sign of a coolant leak and further investigation will be needed.
Throttle's on the right, so are the brakes. Good luck.