I think the 15w-20 might be your problem
Owners manual for the 650 says 10W30 for riding down to 10F, and 20W40 for riding the bike above 40F.
I use the 20W40 and use the 40F lower temp limit as my sanity check for not riding in the winter.
The lower number is the viscosity reference for when the engine is cold, the higher number for when the engine is hot.
I have 36k miles on my bike, and never have to put oil in between 4000 mile changes. The oil never comes off the high mark in the window for the entire 4000 miles.
15W20 is like water compared to 20W40. You might be burning the oil off blowing past the rings or the valve seals, or it may be foaming up like bubble soap and getting sucked out of the oil breather hose into the carbs.
Pretty much anywhere in the US you should be good with 20W40 from now till the end of fall. I would change it over, just the oil, the filter is good for 8000 miles, and put the breather hose back on the air box.
Having it off the airbox will let it suck outside air, rain, moisture, dust.... into the crankcase. It doesn't just blow oil fumes out, it pumps air out and back in as the pistons move up and down in the cylinders. That's why its connected to the airbox, its behind the air filter.
Its funny, you subconsciously answered your own question with your "another thought". That happens a lot, its like something is simmering on the back burner till it boils over... Ive learned over the years to always listen to that little voice in the back of your head.
BTW, how many miles are on the odo? Until you get well over 60k miles I would not expect the bike to be burning (using) oil at all, well... unless you are riding the bike over 90mph...
Last edited by KCW; 04-25-2018 at 06:39 PM.