its either water or anti freeze, either will do the "chocolate milk" thing to the oil, it wipes bearings also.
anti freeze gets in from a few places, but a common is the intake gasket leaks into the lifter valley, I use silicone around the water ports of the head/intake for many years now because i had this situation happen once. you could pressure test the system, oil drain plug out see if antifreeze comes out the oil pan...
