Not to disagree with what's been said above.... but.....
My bike discharges from an out-turned pipe near the rear of the RFM as Stevens in KTB. If the pipes not turned out properly I get a bit of oil on the side of the tyre after a decent run, otherwise I get a few drips on the garage floor when I park up. After working for BP for a number of years I refer to this stuff on the floor as 'mousse'. Its the cafe creme coloured stuff that's a combination of oil, air, and water. Always all three.
I know where the oil comes from, after reading the above, we all know a little more about where the air comes from. Unless we are in major trouble, the water is condensation.
This 'mousse' is not good stuff to have in an engine, and can be a serious problem in the wrong place (imagine pumping that through a set of plain big-end shells). It doesn't flow as oil, and is unwilling to settle out, once frothed up,for practicable purposes, it stays as mousse . I'm not exactly sure where in our 'discharge systems' the water component becomes significant in the already present oil and air, but as our engines arn't usually filled up with that gloop, its clearly quite a way through the system. A reasonable guess might be actually within the external pipework where its being cooled?
I've come across this mousse in reasonable amounts in round barrel Guzzis (T3/ Le Mans 1), where it is collected in a breather box. On the Guzzis, the engine breaths from the crankcase, gearbox, and the 2 rocker covers; all via external pipes into a tin box about the size of a fag packet. The box contains a ball valve and separator/spill where oil is allowed to dribble back, and a vent to air. On flushing these boxes out they are often found to be full of mousse on apparently healthy motors. Within the motors themselves, I have only ever found the smallest amount of mousse right where the breather hoses join the rocker box covers.
One function of any system of valves, balls, flaps etc needs to be to ensure any mousse stays on the correct side, that's the side away from the motor - and that there is no way back. On plenty of bikes gravity works really well.