Low-cost petrol stations: the water myth and the additives war

Low-cost petrol stations: additives, water and the warning light

A driver had spent months with a strange noise in his engine and the check engine light on. From garage to garage. Nobody could find the problem. Until an old-school mechanic asked him the question nobody else had asked: "Where do you buy your petrol?" The answer — unbranded stations, the cheapest ones — was also the diagnosis. Two months later, after filling up at a well-known brand, the noise disappeared and the warning light went off on its own. He never went back.

The story, told in the first person, sparked an exchange of nearly a hundred replies mixing hydrocarbon chemistry, urban legends and savings calculations. The underlying question: does bad petrol really exist, or is it a tale that the big oil companies have been feeding for decades?

Where the petrol you put in your tank comes from

The short answer is that almost all the petrol and diesel sold in Spain comes from the same place: the CLH storage network, the former hydrocarbon logistics company. Product arrives there from the refineries, is stored and distributed to service stations, whether Repsol, BP or the local supermarket. The base fuel is the same.

What changes is the cocktail of additives each brand adds to that fuel before it comes out of the pump. Repsol, BP, Cepsa and the like boast of their own formulas which, according to their marketing, clean injectors, reduce consumption and extend engine life. The unbranded chains, in theory, add fewer additives or none at all. Some claim Shell and BP are the ones that add the most; Repsol would come behind, and the unbranded brands would close the list.

The proportion of those additives is tiny. A calculation circulating in the industry puts it at around 200-255 millilitres per cubic metre of fuel. Translated into the tank of an average car, we're talking parts per million. Enough to notice the difference? That's where the disagreement begins.

Water in the tank: myth, leakage or condensation

The most repeated accusation is that cheap petrol stations dilute fuel with water. It's a hard claim to sustain: water and petrol don't mix, and a diesel or petrol engine injects fuel, not an emulsion. What is another matter is contamination through leakage or condensation in old, low-turnover tanks.

There is consensus on that point: a station with high turnover renews its product almost daily and has less time to accumulate sediment, condensed water or suspended dirt. A neighbourhood station with little traffic can take weeks to empty its tanks. The problem isn't the brand, it's the age of the fuel and the condition of the facilities.

Some point to an additional factor: fuel tanks that travel by ship and are discharged at the end of the journey, with more suspended residue, end up on the market at a discounted price. It's not a documented practice, but it circulates as an explanation for why two stations of the same brand can deliver different performance.

How much you really save with cheaper petrol

The calculations used in the industry are less spectacular than the illuminated sign suggests. A recurring calculation: if the price difference is 0.2 euros per litre and a car consumes 6 litres per 100 kilometres, the real saving is around 4 cents per litre. For an average driver, that translates into about 40 euros a year.

Forty euros. Less than a basic service costs. The figure dismantles the aggressive savings argument, but also that of those who defend paying the brand premium without blinking. The full calculation, broken down item by item, yields a difference that surprises anyone expecting savings of hundreds of euros.

What is true is that differences between stations can exceed 10 cents per litre in some areas, and there the annual saving shoots up. It depends on the postcode, not the brand.

The additives war: do they protect the engine or is it marketing?

Repsol's trade association defends that its additives are approved by the ACEA, the European Automobile Manufacturers' Association, and that using its fuels does not void the vehicle's warranty. It's a weighty argument for anyone with a car under warranty who doesn't want surprises at the dealership.

On the other side, the response is blunt: there is no independent study proving that these additives extend engine life beyond what the advertising brochure promises. A chief engineer at a Spanish refinery is said to have put it bluntly: additised diesels and petrols don't improve performance. The claim is unverified, but it sums up the sceptical stance.

The argument gaining weight over time is another one: more than the brand, maintenance matters. Changing the oil when it's due, replacing the diesel filter and bleeding it properly. Without that, it doesn't matter what you put in the tank.

The experience of those who drive the most miles

The testimonies of drivers with hundreds of thousands of kilometres behind them are the best thermometer. A 110-horsepower diesel with 285,000 kilometres, filled up almost always at a supermarket station, without a single injector problem. A Toyota diesel with 350,000 kilometres, always on the cheapest diesel available, zero breakdowns. A PSA 1.9D engine with 250,000 kilometres, fuelled 95% at the same unbranded chain, and the only notable breakdowns: two glow plugs.

At the other extreme, cases of cars that start to judder after filling up at a particular station and recover when switching pumps. Correlation doesn't prove causality, but it feeds suspicion.

The conclusion that prevails among those who drive the most: the brand matters less than the station's turnover. A station with daily queues constantly renews its product. One with little traffic accumulates old fuel. And in the end, that weighs more than the logo on the pump.

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The warning light that went off on its own after two months of filling up at a particular brand still has no technical explanation. It may have been the change of petrol. It may have been chance. It may be that the old-time mechanic knew something the official dealership's computers don't detect. With 40 euros a year at stake, the war between unbranded brands and traditional oil companies isn't decided in the engine, but in the wallet.