I just tested synthetic (RP 75W-140) and conventional (Pennzoil 80W-90). I mixed with water in a glass container and put into the microwave until it started to boil. I noted that the RP did show signs of "white crap" in the oil while the conventional only showed traces that could have been left from the RP. I also noted that the RP mix boiled in about 40 seconds while the conventional took a minute plus (the mixes were not identical amounts, but close). Also, the RP was more difficult to clean out of the glass container, it "sticks" or"coats" better. Definitely not scientific, but I think that it is plausible that the synthetics don't handle water/heat as well as conventional.
Hopefully someone else with give it a try and post up!