Why would it be a bad idea?
The link Peter posted expressed my reservations quite well. particularly for a car that may not get used every day in the wet months.....
"pure ethanol is hygroscopic; it attracts water, to the point that it will pull it out of the air. Ethanol and gasoline will mix, but ethanol, gasoline and water will not; the ethanol-water mixture will come out of solution and settle on the bottom of your tank. Add a little oxygen to the mix, and you get rust. However, the more common side effect of this is more immediate; turn the car on, and the fuel pump will draw the water from the bottom of the tank into the engine, where it will promptly kill it, and require a costly dry-out process."
"https://chemistry.stackexchange.com/questions/4270/is-there-a... " I believe.
All true. Plus the ethanol in petrol will destroy some old style fuel hose, as you often see with garden power equipment like hedge clippers and chain saws.
But how likely are you to get a pool of water / ethyl alcohol mix at the bottom of your tank and how likely is it to be drawn into the engine. My experience is that its unlikely. The most probable causes of water in your fuel tank are condensation over winter best avoided by storing the car with a full tank, and water drawn in from the fuel station petrol tanks. Both apply just as much to high octane fuel.
There is another issue. Ethanol has a higher octance rating than petrol and so is used in some race fuels. Tesco have said that they use it in their 99 octane fuel and since Tesco fuel comes from the same refinery as Shell and BP etc , its possible to be in their high octane fuels too. Probably only at the 5% level but I could find no information to confirm or deny this.
Last point. In boating we have a problem of what is called diesel bug, which is a bacterial growth that occurs at the interface between water and diesel fuel at the bottom of the boats tank. The water gets in in just the same way as described above and bio diesel is just as bad / good as alcohol petrol in attracting water. The bacteria dies off, and the resulting sludge blocks pipes and injectors. The aviation industry have a similar issue at small airfields apparently, and the only thing that makes it uncommon with motor fuel is the consumption rate ie the turnover of fuel in petrol station tanks. I havent seen it happen in cars in storage but I would guess that it could. Best either complete;ly empty the tank before the car goes into store or completely fill it to minimise condensation
