Older fiberglass tanks. Certain resins used in older glas tanks are dissolved by ethanol and redeposited in engine innards, causing stuck valves, valve/piston interference, etc.
There are also rumors that during switchover from MTBE oxygentaed fuel (which was mandated here in the US a few years back) to ethanol (which is being used to 10% in place of MTBE to meet emissions standards), the two oxygenators don't play nice together and can cause sludging.
Other than that, there is a slight risk on older fuel systems of the ethanol dissolving older water soluble deposits that were not affected by pure petroleum fuel, causing clogged filters.
jky
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