Here's a paragraph from one of the links I posted prior to this, regarding high alcohol content in fuel.
"The problem is due to the mixing of oil with the fuel for lubrication of the engine. When oil is added to a fuel which has separated into gasoline and water-alcohol layers, the oil only goes into solution with the gasoline. If engine is using a pre-mixed fuel supply, this means all the lubrication is in the gasoline, and none is in the water-alcohol solution. When the engine draws the water-alcohol fuel off the bottom of the tank, not only is it getting a rotten fuel, it is also getting no oil at all. No oil means no lubrication. This increases the potential for damage."
On new machines/fuel tanks I have found you get a grey sludge like gunge which, I believe, may have something to do with the processes used in the manufacture of the tank which reacts the first few times with the fuel. Once the tank has been seasoned this no longer happens.
To help mimimise any potential problems, remove any unused fuel from the tank and filter the fuel (with the type of filter that also removes water) when refilling. You can buy these filters cheaper from a Chandlers.
Kev