Alex Robbins is contributing editor at Telegraph Cars where, as well as responding to readers’ queries, he also contributes reviews of new and used cars, together with articles on buying and selling. 

His knowledge of the used car market informs his many buying guides relating to the best buys in particular sectors, with an emphasis on value for money.  Every week he will answer your questions on buying and selling, as well as solving your car problems, whether consumer or mechanical.

Do you have a motoring dilemma you’d like our expert to solve?  For consumer and used car advice, or car faults, email CarsAdvice@telegraph.co.uk and include your subscriber number. This week’s question…

Dear Alex…

My neighbour travels to Spain in a couple of weeks for his annual four-month sojourn. He has been advised to drain his lawnmower’s tank of the new E10 fuel and not store any in his can, as it emulsifies, but this is impossible with a car. What should he do about the fuel in its tank?

– AW

Dear AW…

You’re correct that there’s a lot of concern at the moment about E10 fuel (denoting up to 10 per cent bioethanol content, which became the standard regular unleaded fuel in September). One of the main worries is that ethanol is extremely hygroscopic so, in theory, if E10 fuel is left in a tank for a long time the ethanol content of the fuel will absorb water from the air that’s also in the tank.

Not only is water in fuel bad for the car’s engine, but it can result in a phenomenon called phase separation, whereby the ethanol in the fuel absorbs so much water it comes out of suspension with the petrol, resulting in a low-octane, poor quality fuel and a diluted layer of alcohol sitting beneath it in the tank.

I asked Simon Williams, the RAC’s fuel spokesman, about these fears. He said: “While ethanol is hygroscopic, the octane rating won’t be affected as E10 contains stabilisers to ensure the water stays in suspension.”

On that basis, your neighbour’s car should theoretically be fine if it’s left with E10 fuel in the tank. But if he still isn’t certain, or wants to make doubly sure, there are a couple of extra precautions he can take.

The first is to run the car low before he leaves, then brim the tank with superunleaded. This is still sold at the old E5 grade, which can contain up to five per cent ethanol – some fuel companies’ superunleaded products don’t even contain that much. What’s more, if the tank is as full as can be, there should be less air with which the fuel can react, thus reducing the likelihood of any problems.

The other option is to add a fuel stabiliser of his own. Several brands are available, none of which I’ve tried myself, but a quick Google search for “E10 stabilisers” will give him all the options. For what it’s worth, he can take these precautions with his lawnmower, too – it should prevent him from having to drain the tank and dispose of perfectly good fuel.

If he’s really worried, he could always leave a sample of the same fuel in his lawnmower’s fuel can while he’s away. When he gets back, if the fuel in the can has split, the chances are the fuel in his car will have done so too. At least that way he’ll know not to start it and to get the tank siphoned and refilled with fresh petrol first.

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