What is E10 fuel and can I use it in my car?

For these older cars, experts have warned that the increased ethanol content is likely to lead to issues in the long term, the most common being blocked fuel filters, damaged fuel pumps, the rapid degradation of fuel lines and corroded carburettors.

However, the government has confirmed that owners of these cars will be able to purchase less problematic E5 fuel, albeit in more expensive, higher-octane super-unleaded form.

The Petrol Retailers Association said in 2021: “E5 will still be available in five years' time, but only as the protection grade in ‘super’. It will be reviewed in five years' time.”

Should I use E5 or E10 petrol?

If your petrol car is unable to run on E10 (see above), then E5 fuel is still available, albeit in more expensive (and higher-octane) super-unleaded form.

But if your car’s user manual says it's compatible with E10 fuel, then there’s no reason why you shouldn’t use it. Apart from a drop in fuel economy compared with the E5 alternative, there should only be a minimal difference at all in the way your car drives. 

Which cars cannot run on E10?

Most petrol cars manufactured since the 1990s are able to run on E10 fuel without issues, but there were some exceptions – particularly during the early 2000s.

The government’s website hosts an E10 compatibility checker, which asks you to enter the manufacturer of your car.

It will then tell you which petrol cars from that brand are compatible with E10, and which are not. For example, entering Alfa Romeo tells you that all petrol engines used in the Mito supermini are compatible with E10. However, only a select few in the 159, Brera and Spider can run on it.

If you still aren’t sure after checking online, look inside your car’s fuel filler flap or check the owner’s manual. Failing that, ask the manufacturer, or consult a specialist forum such as an owner’s club.

Since 2011, all new cars sold in this country have had to be E10-compatible.

What happens if I put E10 petrol in my old car?

If you make a mistake at the pumps and brim your older car with E10, all is not lost. Unlike the fuel-tank draining consequences of a petrol-diesel misfuel, simply dilute it with E5 from then on and it should be fine. But don’t make a habit of it, say the fuel manufacturers, including Shell.

E10 can affect older cars in the following ways:

Cold start

Higher ethanol content in petrol can make it harder to turn over an engine from cold.

Vapour lock

Ethanol’s higher volatility can contribute to vapour lock (petrol becoming gaseous) when operating temperatures are higher, causing stalling.

Leaks

Ethanol’s high solvency can cause problems with many seal and gasket materials that are used in fuel systems, as well as with glassfibre resins.

More leaks

Besides a risk of fuel leaks, rubbers and resins can get partially dissolved, producing deposits that could foul carburettor jets.

Corrosion

Ethanol can become acidic and cause corrosion of aluminium, zinc and galvanised materials, as well as brass, copper and steels coated in lead or tin.

Should I use E10 petrol in my sports car?

Many performance cars – the BMW M5, Mercedes-AMG A45 and Porsche 911, to name a few – are designed to run on high-octane fuels, typically demanding 98 RON or better. Currently, fuels with these octane ratings are still E5 blends.

That isn’t to say that you can’t run them on E10, but you might notice a slight change in the engine’s power delivery: regular users of super-unleaded often report that their car ‘feels’ more responsive, especially in the low to medium rev ranges. Many independent studies have revealed the modest power gains that can be gleaned from higher-octane fuels, with testers reporting subjective improvements on the road.

If you want to make sure you’re getting the most out of your car’s engine in terms of performance and fuel efficiency, then super-unleaded is still the way to go.

Yes, it’s more expensive to buy, but factor in a slight increase in fuel efficiency over 95 RON unleaded – particularly E10 – and the cost difference might not be as noticeable as you expect.

Does E10 fuel give lower MPG figures?

The US’s Environmental Protection Agency has estimated that E10 can reduce fuel economy by 3-4% compared with fuel that does not contain any ethanol.

This happens because ethanol contains around 33% less energy than pure petrol. So you have to burn more of it to achieve the same power output. 

Nonetheless, if your car’s user manual says it's compatible with E10 fuel, then there’s no reason why you shouldn’t use it. Apart from that drop in fuel economy compared with the E5 alternative, there should be only a minimal difference at all in the way your car drives. If you’re unsure which fuel to use, always consult the car’s manual.

Is it okay to mix E5 and E10 petrol?

Yes, it is okay to mix E5 and E10 petrol. The risk of using E10 in a car that is not built to use it is to do with the corrosion of rubber and plastic components, rather than any issues with how the engine runs. This corrosion takes place over an extended period of time, so one-off use shouldn’t cause any problems.

If you fill a car that isn’t E10-compatible with the fuel, the RAC recommends topping it up with E5 after using a quarter of a tank. As long as you don’t make a habit of using E10 regularly in a car that does not support its use, you should be fine.

If you’ve filled a car that isn’t E10-compatible with the fuel and plan to let it sit for an extended period – such as putting it into storage for the winter – make sure you’ve cleared as much E10 from its fuel system as you can using the above method.