Supermarket Fuel vs Branded Fuel: Standards & Additives

Answer first: legally sold UK petrol and diesel must meet the applicable fuel specifications, so supermarket fuel is not automatically “bad fuel.” That does not mean every product has the same octane, ethanol content or additive package. Official UK sampling found only minor supermarket-versus-branded differences in the composition measures it tested, but it did not measure engine cleanliness, power, reliability or long-term wear. A controlled study supports a possible injector-cleaning benefit from a high dose of deposit-control additive in some used direct-injection petrol cars, but it cannot prove that every branded forecourt fuel outperforms every supermarket fuel.

What UK fuel standards do—and do not—guarantee

UK motor-fuel rules use recognised automotive specifications, including BS EN 228 for petrol and BS EN 590 for road diesel. The standards cover properties needed for safe and suitable use, such as octane or cetane-related requirements, vapour pressure, sulphur and biofuel limits.

Meeting the same base specification does not require every retailer to sell an identical recipe. Suppliers can use different refinery streams, ethanol proportions within the permitted label, seasonal blends and additive packages while remaining compliant.

A fuel specification is a minimum product-quality framework. It is not a controlled comparison proving that all brands perform identically, and it is not evidence that one logo is always superior.

What the government-commissioned 100-sample study found

A DESNZ-commissioned report published through the National Atmospheric Emissions Inventory analysed 50 petrol and 50 diesel samples collected in June and July 2023. Samples came from five UK areas and covered four major supermarkets, oil-company-branded stations, regular fuels and available premium grades.

The laboratory campaign measured carbon content, sulphur content and biogenic content. For regular petrol, the report's weighted averages showed only minor differences in the carbon content of the fossil fraction between supermarket and branded samples. For diesel, it described the measured regular-versus-premium and supermarket-versus-branded carbon-content averages as highly similar; weighted sulphur averages were also close.

That is useful evidence against the idea that supermarket fuel is a completely different or inherently non-compliant product. It is not a retailer performance test. The study did not measure:

power, acceleration or throttle response;
fuel economy in vehicles;
injector or intake-valve cleanliness;
engine wear, reliability or repair rates;
the exact detergent dose in every retail product.

The authors also identify limits: sampling was summer-only, the number of sites was constrained and the weighted results were intended mainly to improve national emissions factors rather than rank forecourt brands.

Can additive packages differ?

Yes. The same DESNZ-commissioned review says branded petrol may contain performance additives such as octane improvers or detergents, and reports an industry view that supermarkets may use lower treatment rates. That passage is based partly on correspondence with Fuel Industry UK rather than an independently controlled retailer-by-retailer additive analysis, so it should not be turned into a blanket claim.

Additives can serve different functions, including deposit control, oxidation stability, corrosion protection and conductivity. “More additives” is not a complete quality measure: chemistry, dose, engine design, existing deposits and the outcome being measured all matter.

What controlled additive testing can tell us

A 2024 SAE technical paper tested seven pre-used gasoline direct-injection vehicles. In its clean-up phase, the vehicles ran for at least 4,750 km on EN 228-compliant petrol treated with a high dose of deposit-control additive; two vehicles exceeded 5,800 km. The researchers reported fleet-average reductions in particulate emissions, fuel consumption and tailpipe CO₂ during continued use.

The second phase alternated otherwise identical additivated and non-additivated fuels. It found no statistically significant instantaneous difference between them. The authors therefore attributed the longer-run result to removal of existing injector deposits rather than an immediate energy effect from adding detergent to clean fuel.

Important limitation: every listed author was affiliated with Shell Global Solutions and the work was copyrighted by Shell. The study is relevant experimental evidence, but it does not independently rank UK retailers, and a high-dose test fuel is not proof that every premium pump product produces the same result in every car.

Vehicle-level results also varied. Some engines had more opportunity for deposit clean-up than others. That makes broad claims such as “premium fuel always adds MPG” or “standard fuel can never affect cleanliness” too strong.

Octane and detergency are different questions

RON describes resistance to knock. Deposit-control additives address cleanliness. A 99 RON fuel may also have a premium additive package, but the octane number alone does not tell you the detergent concentration.

Use the minimum grade required by the vehicle manufacturer or ECU tuner. A car calibrated for 95 RON may show little economic return from a higher-octane fuel, while a performance or remapped engine can require or make better use of a higher grade. See the separate RON 99 guide for the octane decision.

Are supermarkets always cheaper?

No. Supermarkets often compete strongly on price, but the cheapest station changes by area, fuel grade and time. The CMA's August 2026 monitoring continues to compare margins and pump prices across supermarket and non-supermarket retailers; it also reports that price variation and competition remain important concerns.

Compare the actual grade and current local price rather than relying on the retailer category. A supermarket stop can lose its advantage when it requires a queue, a retail-park loop or several extra miles. A branded station directly on the route may be the lower total-cost stop even at a slightly higher pump price.

Your situation	Practical decision
The handbook requires 98 or 99 RON	Use the required octane; compare like-for-like premium products and current prices.
The car requires standard 95 RON E10	Choose a compliant, correctly labelled fuel; compare price, freshness and route before paying a premium.
The car is remapped	Follow the tuner's stated octane and fuel requirements, not a generic retailer rule.
You are diagnosing rough running or injector faults	Use proper mechanical diagnosis. Do not assume a different brand is a substitute for repair.
You want to test a premium claim	Compare several tanks, litres added and odometer miles over similar routes; one trip is too noisy.

The Route & Fuel decision

Select the fuel grade your vehicle actually requires.
Compare recent prices for that same grade.
Check when the price last changed and whether the station is open.
Calculate the gross fill-up saving.
Subtract fuel used over the extra distance and consider the extra time.
Treat additive or premium-fuel benefits as vehicle-specific unless a source tests your product and outcome directly.

This approach avoids both extremes: dismissing all product differences and accepting every marketing claim as a guaranteed saving.

Supermarket fuel vs branded fuel: standards, additives and value