That would be a right mess, wouldn't it... Not something you could slurp down before you're even half-awake, for sure. No, what I called "sour milk" is more like yoghurt or kefir, consistency-wise.
Come to think of it, one kind of sour milk is called "churning milk" -- as in, churning butter -- in both Swedish and Finnish, and that doesn't seem too different from the other kinds, to me; so it might well be that they're all something like the "butter milk" BOx was on about.
Or you can of course use ordinary regular bog-standard normal milk with your cereal. Our Matsku, for instance, usually pours that over his Wheeties or Cheerios or whateverthefuck those vaguely-grain-based sugar-and-chemicals calorie-bomb concoctions are called.