Used cooking oil is being used by some owners of older diesel vehicles as a cheaper alternative to diesel fuel, despite being officially banned. With diesel fuel priced at over 2.35 euros per liter, the practice is attracting motorists who own older diesel vehicles, a practice that is still officially banned due to the absence of a decree. Yann Deva is one of these enthusiasts, attracted by both economic arguments and the idea of recycling used oils instead of burning petroleum. Based in Ariège, he says he has traveled about 60,000 kilometers using cooking oil. The method involves collecting oil from restaurants, leaving it to settle for a few days so that residues sink to the bottom, then filtering it through one-micron filters to clean away traces of fried food before adding it to the fuel tank as a supplement to diesel. The dosage depends on the season, ranging from 10% to 20% in winter (as cooking oil does not tolerate extreme cold well) to up to 90% in summer. Only older diesel vehicles, preferably those with indirect injection and a Bosch fuel pump, such as the Mercedes 190 or Peugeot 205 models from the 1980s and 1990s, are suitable for this. More viscous than diesel, cooking oil is not well tolerated by the direct injection systems of newer vehicles. Yann Deva warns that the practice requires regular maintenance and getting one's hands dirty. On the island of Oléron, the Charente-Maritime branch of the association "Roule ma frite" advocates for the transformation of restaurant oils into local fuel. With the surge in diesel prices, a consequence of the war in Iran and to a lesser extent the attacks on Russian refineries, the association's phone has been overwhelmed. Grégory Gendre, who founded "Roule ma frite 17" nearly twenty years ago, says there was a significant increase in interest once diesel exceeded 2.40 euros per liter at the pump. Many callers live in the countryside, rely on their cars, and are looking for solutions to ensure that going to work does not cost them money. Some associations collect used oil for free from restaurants and sometimes sell it for about 80 cents per liter to users. The practice currently exposes individuals to sanctions. Any product used as fuel is subject to specific taxes, and in the event of an inspection, customs officials can impose fines on individuals who deviate by supplying themselves with used oil. Yann Deva says one has to be "unlucky" to get caught and cites "legitimate disobedience" as a reason. The legal framework has evolved, with the conditional authorization of used cooking oils as fuel in the 2023 finance law proposed by former ecologist deputy Julien Bayou. However, nearly four years later, the implementing texts are still missing. Julien Bayou, who is also a lawyer for "Roule ma frite," regrets that an opportunity to ensure some energy autonomy is being lost, even if it is not a panacea. He intends to take the matter to the Council of State. In 2024, when a decree proposal was under review, the National Agency for Food, Environmental and Occupational Health Safety (Anses) deemed the studies on emissions from these oils rare and contradictory. It suggested reserving the oil for "short and specific value chains," such as tourism trains or certain dedicated fleets. This is exactly the type of use that "Roule ma frite" is advocating for, with Grégory Gendre citing the example of amateur sports club buses whose travel costs have exploded in recent weeks or a small tourist train on the island that intends to operate using oil from neighboring restaurants once they receive authorization. Even if legalized, the oil will only replace diesel on the margin: the national stock of used oils is estimated at less than half a percentage point of the 32 million cubic meters of diesel delivered in France in 2025. The idea is not new. During the 1900 World's Fair in Paris, a small diesel engine was already running on peanut oil, at the request of the French government, according to Rudolf Diesel, the German engineer after whom it is named.