Supermarket petrol quality: what you need to know

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 (and can charge at home), 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? We explain everything you need to know.

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Is supermarket petrol quality worse?

All standard unleaded fuel sold in the UK conforms to the same British Standards, so you shouldn’t consider supermarket fuel to be below par.

The key differences between fuels will come when you compare standard and premium (often referred to as 'super') fuel grades. But keep in mind that even supermarkets have their own premium fuel (such as Tesco Momentum 99) that offer similar claimed benefits to the premium fuels from other brands.

You can read more about the claimed benefits of premium fuel below. 

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Premium unleaded: supermarket vs branded 

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

• Detergents These can prevent or even reverse the build-up of carbon deposits on sensitive engine components.
• Friction modifiers These lubricate the engine’s combustion chambers, with the aim of 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. It's worth noting that the brands we checked tend only to make specific performance claims about their premium fuels, while their standard unleaded don't tend to have claims beyond what you'd expect a standard fuel to have. 

Premium unleaded typically has greater amounts of performance additives and this is the stuff that the brands spend time telling you about in marketing material. For example, Esso claims it has three times more detergents in its Supreme+ fuel versus its standard petrol along with a better way of reducing friction and better protection against corrosion. Tesco is less specific about Momentum 99, simply stating that it has a 'special blend of additives' to keep the fuel system clean. 

Another difference between standard and premium fuel is the ethanol content. Standard unleaded has had 10% ethanol (known as E10) since 2021, while some premium fuels can have as little as 5% (E5) as some older or specialist engines won't work with E10 fuels. 

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What is high-octane unleaded?

Aside from typically higher levels of performance additives, the key difference between regular and premium 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.

This means that premium unleaded can be compressed more in an engine without prematurely igniting. Using normal unleaded in cars requiring higher-octane fuel can cause engine 'knock' – this is 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 for your car. However, 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 petrol 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.