The theory that rings the most true is that there's an air leak on the suction side of the oil pump. The oil uptake tube that hangs down low in the oil pan -- the section of this tube that's exposed to air -- probably where this tube connects at the top....it should have an o-ring to make a good seal. If there's a leak there, then crankcase gas (which is air with high water vapor content) gets aspirated into the oil before being pumped forward. Would explain the froth.

Past the oil pump, the oil is under positive pressure, so any leak will result in oil dribbling out, not air sucking in. Air aspiration can only happen on the suction side (between the oil in the pan and the pump).

To investigate, drain the oil pan, remove oil filter. Remove the oil pan. Make sure the oil uptake pipe is properly sealed at the top.