Hi Tyler, Yep I've seen that happen many times before. Fuel today has a very short shelf life and it turns into all kinds of milky gunk as you describe. I have even seen it turn into something resembling water. I've often had to thoroughly clean up tanks and carbys after allowing fuel to sit for a short time. I have also noticed that silt-like stuff but can't explain it. Perhaps as you say, some kind of additives are unstable and mineralize as the fuel slowly evaporates. It can also turn green as I'm told a special kind of algae can start growing in it.
And another thought just came to mind. Do you have kids? One time a neighbour brought his mower over telling us he couldn't start it. He said it usually started first pull. When we looked in the tank and smelt it, we soon realized what was wrong. The fuel tank had been half filled with water! We told him he must have left it out in the rain. "No" he replied, "always put her in the shed!" We cleaned it all out and put it back together, put some fuel in and away she went, first pull. He took it back, still scratching his head. He comes back a day or two later to tell us his 6 year old son had just told him he wanted to help and had got his sand bucket, filled it with water from the hose, and was about to fill up the mower tank... but this time he caught him in the nick of time. So mystery solved!
