It's probably just condensation built up in the crankcase over time and lack of use, possibly because the breather has become restricted and due to the damp conditions on the day. How often is the bike used and I mean ridden for at least 30mins? Is or has your bike been rarely used but you wheel it out once a week/month and start it on the drive to warm it up?
One of the biggest falsies is doing the above, all it does is builds up condensation and over time this blocks the breather system, holding the water vapor/moisture in the crankcase, which in time emulsifies, turns white and collects in the sight glass or on the oil filler cap.
Check the coolant expansion tank, if it's oily, black/brown it's likely the head gasket has gone, check the rad for the same, if it's clean and no oily smell or residue on the surface, it's not a blown head gasket. Check the crankcase breather system, look up the pipes if it's thick with emulsified oil (mayo) clean it all out and I mean
all of it from the whole system, if it's just slight, clean the pipes and any trapped elsewhere should just burn off over time. Give the bike a really good run like couple of hours, do that a few days on the trot, the problem should disappear, if it doesn't you've got a likely head issue or other issue just starting.
The first two of other possibilities your chap mentioned would give you serious other issues, like overheating and the emulsification would be bad so very unlikely in my experience. The last wouldn't do anything to the oil just overheat due to lack of pressure and serious loss of coolant.
Hopefully the problem is just condensation build up, moral of the story don't wheel out the bike every week or so and start it, hook it up the battery charger every few weeks or so and just leave it alone.
