Help me diagnose why my head gasket blew

I think in terms of water in the fuels - that's a side affect of the ethanol mix (10% here in CT). So it's not refining, but time int he ground. Older smaller places have older tanks in terms of both technological and sealing quality, and the lowest grade (cheapest) moves the quickest from tanker to cars' tanks. I will only get fuel from a few big name (Mobil or Shell) places - all have had tank replacements within 5 years (we've had the 10% blend at least over the winter for about 8 years now). Up here tank replacement is mandatory after 15 years. I also only run premium fuel and I would swear the cheaper places (7/11, Citgo, etc) have a higher percentage of ethanol. They smell much different and I lose about 2mpg running them even at the same octane.