Debunking Premium Gasoline Myths: Engine Cleaning Claims Face Scrutiny

Premium gasoline, commonly known as "high-octane" or "haioку" in Japan, has long been associated with superior performance and engine cleanliness. However, a significant 2020 investigation by the Mainichi Shimbun revealed that many of these claims were misleading. The newspaper reported that premium gasoline products, which had been advertised as containing effective detergents for engine cleaning, did not deliver the advertised benefits consistently. Moreover, some oil companies were misrepresenting octane ratings, and gasoline supposedly developed independently by each company was actually being sourced and mixed from the same suppliers across the industry.

Following the 2020 scandal, major oil retailers modified their marketing claims. ENEOS now states that its premium gasoline contains synthetic detergents that prevent carbon deposits from adhering to engine intake systems—a more cautious claim that stops short of promising to remove existing deposits. Costco similarly clarifies that detergents are added to both its premium and regular gasoline formulations. Despite these corrections, some consumers continue to use premium fuel in regular-octane vehicles hoping for engine cleaning benefits, a practice lacking strong scientific support.

The detergent most commonly cited for high cleaning performance is PEA (polyetheramines), a component found in aftermarket fuel additives. However, PEA-based products require precise dosing instructions to be effective, and overdosing can backfire by leaving unburned residues that contaminate engine oil. When mixed with dissolved carbon deposits, these residues can degrade oil quality and potentially damage engines if oil changes are not performed promptly. Given the caution required when using such additives, it is questionable whether mainstream premium gasoline products contain sufficient concentrations to deliver noticeable effects.

For drivers seeking genuine engine cleanliness at optimal cost-effectiveness, specialist fuel additives used according to manufacturer specifications may prove more beneficial than relying on premium gasoline alone. For example, a vehicle averaging 10 kilometers per liter and traveling 1,000 kilometers monthly would face approximately 1,000 yen in monthly price difference between regular and premium fuel—roughly 12,000 yen annually. Allocating that budget toward high-quality aftermarket detergents and timely oil changes would likely yield superior results compared to the uncertain benefits of premium fuel.