Interesting discussion. As part of the STC I got for the Auster to use Mogas there was a formal method for checking whether petrol has ethanol. Fill a container with water up to a line then add fuel and shake. The ethanol will mix with the water and increase the volume of that layer - separated out from the fuel - and you can directly tell the %ethanol from that as well. So, as far as this test is concerned the ethanol does not pick up the water from the tank.