What’s behind the warning about supermarket fuel?
Most British drivers fill up where it’s cheapest, and that usually means a supermarket forecourt. A veteran mechanic, Reaz, who has run his own garage for 17 years, says the savings can come with a quiet cost: fewer detergent additives than you’ll get from big oil brands. His advice, shared in coverage by the Mirror, is simple—don’t rely only on budget fuel and alternate between premium and regular to keep engines cleaner for longer.
Reaz’s point is about deposits, not octane bragging rights. Additive packages—detergents, corrosion inhibitors, anti-foam agents—are mixed into fuel after it leaves the refinery. Branded fuels from BP, Shell and Esso are marketed with stronger additive bundles aimed at reducing intake-valve and injector deposits and maintaining fuel economy over time. Allied Autocare echoes that view, arguing that “you get what you pay for” when it comes to additized fuel.
Supermarket chains reject the suggestion that their fuel is harmful. Their stance is consistent: all fuel sold on UK forecourts must meet British and European specifications. They say they comply, period. That’s backed by oversight from UK authorities and trading standards, who check fuel for compliance with legal specs like octane, sulphur and ethanol content.
So where’s the gray area? Standards set the legal minimum for properties such as RON/MON, sulphur content and oxygenates. They don’t force retailers to use a specific level of deposit-control additives beyond what’s required to meet those specs. That leaves room for brands to go above the baseline with proprietary packages—and for cheaper outlets to add less, if they choose. The base fuel often comes from the same terminals; it’s the additive cocktail that tends to differ at the loading rack.
This isn’t the first time UK drivers have worried about what’s in the pump. There have been rare, highly publicised incidents—like contamination cases years ago that caused sensor issues—showing how sensitive modern engines can be to the wrong chemistry. Those events were exceptions, resolved with recalls and compensation, but they left a mark on public trust. Today’s debate is less about contamination and more about whether long-term cleanliness justifies paying extra.
Do additives and premium fuels really help—and should you switch?
Detergent additives are designed to stop gunk building up on intake valves, injectors and combustion chambers. Over tens of thousands of miles, deposits can skew air-fuel mixtures, sap performance, and nick fuel economy. Stronger packages aim to prevent those deposits and sometimes clean up mild ones. In petrol engines with direct injection and turbocharging, deposit control matters more, because the injectors and intake tracts run hotter and tolerate less buildup.
Premium petrol brings two things: more detergents (in many cases) and a higher octane rating. Octane (RON) is about knock resistance, not power by itself. If your engine is designed for 95 RON, you won’t turn it into a sports car by using 99 RON, but you may reduce knock under heavy load or hot weather and keep things cleaner over time. For engines that recommend or require higher octane, using it isn’t optional—it’s essential for longevity and performance.
Diesel is different. There’s no “octane”; instead, premium diesel usually raises cetane and adds detergents, demulsifiers and anti-foam agents. Benefits can include quieter running, smoother cold starts and cleaner injectors, especially on high-mileage or urban-driven cars. Again, results vary by engine and driving pattern.
What does the evidence say? Independent tests over the years have been mixed. Some have found small improvements in fuel economy after extended use of fuels with stronger detergents; others have shown little day-to-day difference you’d notice on one tank. The real gains tend to show up over time—fewer deposits can mean fewer drivability issues and more stable MPG, especially in engines prone to buildup.
Cost matters. The gap between standard and premium can be 8–15p per litre. On a 50-litre fill, that’s £4–£7 extra. If your car doesn’t need higher octane, you might not want to pay that every time. That’s where Reaz’s rotating strategy comes in: alternate between regular and premium, or run a few tanks of a top-tier additized fuel every few thousand miles. It’s a middle ground that aims to control deposits without doubling your fuel bill.
There are also practical steps that don’t involve changing stations. Busy forecourts often have fresher fuel, which helps with quality consistency. Keep receipts, especially if your car develops a problem right after a fill. Stick to your manufacturer’s minimum octane and fuel type—E10 petrol (95 RON) is standard; E5 premium is widely available for older or incompatible models. Short-trip, stop-start driving accelerates deposit formation, so an occasional longer run at operating temperature can help too.
Where does this leave UK drivers? Here’s a simple framework:
- If your owner’s manual calls for higher octane, use it—every time.
- If your car is knocking, idling roughly, or losing MPG and you mostly do city miles, try a few tanks of premium with strong detergents.
- If you’re happy with how your car runs on budget fuel, consider an occasional “clean-up” tank and regular servicing of plugs, filters and injectors.
- For diesel owners with rough starts or noisy idle, premium diesel can help, but give it a couple of tanks to show results.
The bottom line is less dramatic than the headlines. All legal fuel on sale in the UK must meet strict specs, and cases of outright damage from normal, compliant supermarket fuel are rare. But additive packages do differ, and that can matter for how clean an engine stays over time. For many drivers, a hybrid approach—buy where it’s convenient and priced right, then rotate in a premium or branded tank periodically—balances cost with care.
