Is Supermarket Petrol Bad for Your Engine? The Truth

The legal standard all fuel must meet

In the UK, every litre of unleaded petrol sold at any pump — whether at a rural BP or a suburban Asda — must conform to EN228, the European standard for unleaded petrol. This covers octane rating (minimum 95 RON for standard grade), sulfur content, benzene limits, ethanol content (E5 or E10), and a range of other chemical properties.

There is no legal mechanism by which supermarkets could legally sell "lower quality" fuel. If it's in the pump, it's passed the standard. The Trading Standards Agency and the DVSA regularly test samples from forecourts across the UK. Non-compliant fuel is extremely rare and, when found, results in immediate closure of that forecourt and legal action.

Where does supermarket fuel actually come from?

This is where the myth really falls apart. The UK has a relatively small number of oil refineries — Grangemouth, Fawley, Humberside, and a few others — and they supply fuel to the entire country via a shared pipeline network.

A tanker leaving Grangemouth might deliver to a Shell depot, an Asda depot, and a Texaco depot from the same batch of refined fuel. The fuel is essentially the same product at that point. What happens at the depot is where minor differentiation occurs — some operators add proprietary "additive packages" to their fuel, but for standard grades, the base product is the same.

The idea that Asda is sourcing some inferior, budget crude from a sketchy refinery and Shell is using a superior, magical blend is simply not how the supply chain works.

Where branded fuel genuinely differs: premium grades

Here's where we give credit where it's due. Premium fuel grades — Shell V-Power (99 RON), BP Ultimate (97 RON), Esso Supreme (99 RON) — genuinely contain proprietary additive packages that standard supermarket fuel doesn't. These typically include:

Detergent additives that clean fuel injectors and intake valves over time. An engine that has used premium fuel for years may have cleaner injectors than one run exclusively on basic fuel.

Higher octane (95 RON vs 99 RON). A higher octane rating allows a turbocharged or high-compression engine to advance its ignition timing, extracting more power and potentially better fuel economy. This is a real benefit — for the right engine.

But — and this is crucial — most cars don't benefit from 99 RON fuel. Unless your car's manual specifically recommends or requires premium grade, your engine is tuned for 95 RON. Putting 99 RON in it won't hurt, but it won't meaningfully help either. You'd be paying 12–18p/litre more for no practical benefit.

The origin of the myth

So why does the myth persist? Several reasons:

Brand marketing.Shell and BP have spent decades positioning their fuel as superior. "Made to go further" and similar straplines plant the seed that cheaper alternatives are inferior. It's effective marketing, not engineering fact.

Confirmation bias.Someone fills up at an Asda after a difficult journey, notices their car doesn't feel quite right, and blames the fuel. The actual cause might be a cold engine, worn tyres, or simply a bad day of traffic — but the supermarket petrol takes the blame.

Genuine variation from 20 years ago.Standards and enforcement have improved significantly. Anecdotes from the 1990s or early 2000s about poor-quality supermarket fuel may have had some basis then. They don't now.

The bottom line: fill up wherever is cheapest and genuinely convenient for your car. WorthThePump will tell you which nearby station offers the best real saving. Try it free.