Water...is the only thing I know that will get oil that color, and I have never seen it in a crankcase. I've seen it in a gearbox, differential and transmission.
I would like to know how it is you've seen it before in older engines as I don't see what the age of the engine has to do with it.
Could it be that condensation is forming in your crankcase? It gets hot enough to boil water in there. I just don't see how it could be condensation unless you have extremely high humidity.
Did you take the hose to it or leave it out in the rain? Roof leak?....not likely, unless you left the cover off of the crankcase....sabotage? Got problem with your neighbor, wife jealous of your new mower?
Maybe it some kind of break-in oil?
That is an air cooled engine right?