This article is from the Ducatis Motorcycles FAQ, by Duke Robillard firstname.lastname@example.org with numerous contributions by others.
It's a little condensation, an emulsion of oil and water, the byproduct of
combustion. For every gallon of gas you burn, you'll receive free of charge
roughly one and a half times that back in water, and unfortunately a little
can make it into the oil supply one way or another. It sort of goes away
when you change your oil, and then comes back again. The problem is that it
is very noticeable in the Duck's sight glass. You don't notice it the same
way on a dipstick.
This is why it's so important to ride for extended lengths, particularly
Ducatis as they seem to take a while to heat up their oil. About ten miles
or so at highway speeds is mentioned as a figure. Some folks swap their oil
after this just to be on the safe side, but it may not be needed unless
it's really excessive. 'Course you can always ride further and/or