It's one of the most persistent bits of forecourt folklore: cheap supermarket fuel must be doing something nasty to your engine. Plenty of drivers will happily save money on groceries but drive past a supermarket pump to fill up at a big-brand station.
The short answer is no, supermarket fuel isn't worse. It has to meet the same rules as any other fuel sold in the UK. The differences between brands are mostly in the additives, and those matter far less than the price on the board.
The short version
| Supermarket standard fuel | Branded standard fuel | Branded premium fuel | |
|---|---|---|---|
| Meets UK fuel rules | Yes | Yes | Yes |
| Additives | Varies by retailer | Varies by retailer | Extra detergent (cleaning) additives |
| Suitable for normal cars | Yes | Yes | Yes |
| Usually the cheapest | Often | Varies | No |
The rules every forecourt follows
All petrol and diesel sold in the UK has to comply with the Motor Fuel (Composition and Content) Regulations 1999, as amended, which set environmental specifications for both fuels. The 2021 amendments refer to the two key industry standards for testing:
- BS EN 228 for unleaded petrol
- BS EN 590 for diesel
Strictly speaking, as a government paper on E10 points out, the legal requirement is the regulations rather than the British Standards themselves. In practice, the RAC puts it plainly: "Supermarket fuel conforms to the same standards (the British Standard) as branded fuels supplied by one of the major oil companies."
So a litre of E10 from a supermarket and a litre of standard unleaded from an oil company forecourt are both made to the same specification. Your car is designed to run on fuel that meets it.
So where do the differences come from?
Additives. Retailers add their own additive packages to fuel, and the most heavily marketed differences are in the premium grades, which carry extra detergents designed to keep the engine's fuel system clean.
What premium fuels claim
Premium fuels such as Shell V-Power and BP Ultimate are sold on the strength of their extra additives (and, for petrol, a higher octane rating than standard 95 octane E10). Here's what the companies themselves say:
- Shell V-Power unleaded: Shell says it contains three times more cleaning molecules than its regular unleaded.
- BP Ultimate Unleaded: BP says it removes dirt from inside your engine and helps stop it coming back, and claims "up to 375 more miles per year". The small print says that figure assumes continuous use over 6,400 miles a year, compares against 95 octane petrol, and that benefits "may vary due to factors including vehicle type and condition, driving style and driving conditions".
Note the wording: "up to", and benefits that build up "with use over time". These are not claims you'll notice after a single tank.
What the motoring organisations say
The RAC says premium fuels with detergent packages are designed to keep engine internals, especially fuel injection systems, clean. But it adds that any efficiency gain from higher-grade fuel "may not be significant compared with other factors which affect fuel economy, most notably driver behaviour, vehicle load, tyre pressures and traffic and weather conditions".
Which? says super unleaded typically costs 10p to 15p a litre more than normal unleaded, with "usually little benefit", unless you drive a high-performance or imported car that specifically needs a higher octane rating. It also notes that larger supermarket chains "still generally offer the lowest forecourt prices".
Why is supermarket fuel often cheaper?
It isn't because the fuel is worse. It's about how supermarkets price it.
The CMA looked closely at this in its road fuel market study, published in July 2023. It found that:
- Supermarkets remain, on average, cheaper than other types of retailer.
- Supermarkets have historically used cheap fuel to bring shoppers in. The CMA described this as the "halo effect", where selling fuel cheaply attracts customers who then do their grocery shopping in-store.
- Some supermarkets told the CMA that this halo effect had declined, which weakened their incentive to keep fuel prices low.
- Asda, and to some extent Morrisons, had historically been price leaders, but had taken a less aggressive approach to pricing. Supermarket fuel margins rose from 4.6p a litre in 2019 to 10.8p a litre in 2022.
Supermarkets also sell a lot of fuel. According to the CMA's final report, the big four had grown their share of UK road fuel sales to 44% by 2021.
The caveats
- Cheaper isn't guaranteed. Supermarkets are cheaper on average, not everywhere. A local independent or branded forecourt can undercut a nearby supermarket, and prices change through the day.
- Follow your handbook. If your car's manual or fuel filler cap says it needs 97 or 98 octane, buy the grade it asks for. That's about octane, not about supermarket versus branded.
- Premium additives aren't useless. If you like the idea of a cleaner fuel system and don't mind paying, there's no harm in an occasional tank. Just don't expect it to transform your car.
The bottom line
- Supermarket fuel meets the same UK rules as branded fuel. It won't harm a car designed for standard petrol or diesel.
- The real differences are in additive packages, mainly in premium grades, and the motoring organisations are sceptical that most drivers will notice the benefit.
- Supermarkets tend to be cheaper because of how they price fuel, not because of what's in it.
The simplest way to save is to compare prices before you fill up. PlugOrPump shows live E10 petrol and B7 diesel prices for every forecourt near you, supermarkets and branded stations side by side, so you can check the gap near you. Search by postcode or browse fuel prices by town.