Supermarket petrol quality: what you need to know - Which? (2024)

Filling up at your local supermarket petrol station can often be cheaper than choosing a branded forecourt. But should you be worried about exactly what’s going in your tank?

Daljinder NagraSenior researcher & writer

Supermarket petrol can often be cheaper than other forecourts. But is there any benefit to choosing more expensive petrol, or is it exactly the same as cheaper unleaded?

Unless you’ve made the switch to an electric car (or hydrogen fuel-cell vehicle), a regular trip to the petrol station is an inescapable and expensive drawback of owning a car. When it comes to reducing costs, opting for cheaper supermarket unleaded is a convenient way to save a few pounds every time you fill up. But should you be concerned about the quality of the fuel you’re using?

The short answer is no. All unleaded fuel sold in the UK conforms to the same British Standards, so you shouldn’t consider supermarket fuel to be below par.

However, there are small differences between fuels. These could offer subtle benefits, depending on what sort of car you drive.

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Supermarket vs branded unleaded

The key difference between different brands of fuel is the type and amount of performance additives added to the base unleaded fuel.

These include:

• detergents, which can prevent or even reverse the build-up of carbon deposits on sensitive engine components
• friction modifiers, which lubricate the engine’s combustion chambers, reducing wear.

The specific formula of performance additives added to each brand’s unleaded is proprietary to each producer, although even cheap supermarket fuel will contain them.

‘Super’ unleaded products typically have greater amounts of performance additives.

What is unleaded petrol?

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There are three key components that make up a litre of unleaded fuel:

1. Hydrocarbons

A blend of non-aromatic, aromatic and olefinic hydrocarbons makes up base unleaded fuel, known in the industry as BOB (Blendstock for Oxygenate Blending). The specific blend of hydrocarbons controls the fuel’s specific chemical properties, ensuring it’s suitable for use in internal combustion engines.

Hydrocarbons can also be used to alter the octane rating of petrol (for instance, increasing it for super unleaded products).

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2. Ethanol

To help the UK meet mandatory renewable fuel targets, petrol includes a specified amount of ethanol, which can be produced from crops or waste.

In 2021, the proportion of ethanol in regular unleaded rose to 10% (labelled as E10), further helping to reduce vehicle emissions. The older 5% ethanol petrol (labelled E5) continues to be sold to cater for older cars incompatible with E10 fuel. Typically, however, it's usually available only as high-octane super unleaded (more on this in a moment).

3. Additives

Aside from performance additives, which help increase engine longevity, operation additives are also added to all unleaded fuel. These include antioxidants, which ensure the fuel reaches your local forecourt from the refinery as intended.

Should I buy high-octane 'super' unleaded?

Aside from typically higher levels of performance additives, the key difference between regular and ‘super' unleaded is its higher octane rating.

• In the UK, normal unleaded is rated at 95 RON (Research Octane Number).
• Super unleaded is normally between 97 to 99 RON.

Essentially this means that super-unleaded fuel can be compressed more in an engine without prematurely igniting. Using normal unleaded in cars requiring higher-octane fuel can cause engine knock – where the fuel fires before the completion of the compression cycle (you may hear it as a 'ping').

There’s no harm in using super unleaded, even if high octane isn't recommended. But you’re unlikely to detect any benefit unless your car specifically requires it – normally high-performance models and some imported vehicles.

So unless you own one of these cars, you may as well save your money and buy normal unleaded instead. All cars sold in the UK will run on regular 95 RON petrol. But check your car’s handbook or the label inside the fuel filler cap to find out if high-octane fuel is recommended.

Some super-unleaded products also claim to boost fuel economy. But given that they're typically 10-15p per litre more expensive, you’ll see greater gains by adopting a gentler driving style.

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