Yes, that's the main alternate so you'd expect to see that, as you mentioned, in the oil then.
There is another possibility that I've been experiencing on my overly worn car which is exhaust blowing into the coolant system, which heats it up greatly and causes the some evaporation. However, you'd be hearing that enter your reserve bottle and the sound of boiling would be obvious shortly after a ride.
Dragging knee is for the track, and dragging tail is for the lot. --Kane Friesen
When you're in a car, you're watching a movie; when you're on a bike, you're in the movie. --Robert Pirsig
Identity theft is not Fun