Too much water going through the headers, or a malfunctioning Bassett valve inevitably causes water in the oil. (Reversion from the camshaft overlap... usually upon shutdown.)
One seriously hayseed trick you can do to get rid of a small amount of water in the oil is to drastically restrict the water flow going into the motor until it really really really heats up... then the water content will steam off from the breathers and you are usually back to clean oil.