I did one today that was painted in the summer. For some unknown reason a milky liquid appeared whilst I was cleaning. Thought it was something on my brush at first.
Prob painted by a cowboy with water based paint.
Wouldn't make any difference water or oil based. 
Eh how do you figure?
External Water or oil based paint, once dry is waterproof. Bathroom & kitchen paint is all water based. Dulux weathershield is a superb external water based paint. Oil based paint is being, or has been phased out. Look in your local diy store, you probably won't find any. The technology of water based paints has moved on in recent years & is now as good as the older oil based alternative.

Had this conversation with a trade painter in summer, they were painting the wooded & metal frames of a nursing home I have been doing for over 6 years. I always had spots & streaks from certain windows even after all this time. Once they were painted (water based) every window came up perfect! The job is a doddle now!
