Short answer: 95 and 98 refers to the octane index found in unleaded species. Unlike unleaded 95, the unleaded 98 improves driving in particular situations such as acceleration.
For the record…
Today the reference petrol in Europe, the unleaded fuel 95 (also called Eurosuper) appeared on the market in the early 1990s. In France, until 2009, sp98 and SP95 were the only ones on the market before the appearance of the “so-called European petrol” SP95-10, compatible with all vehicles put into circulation from 2000. In the United States, the octane index is slightly different (index of 87 to 93) but the principle explained below remains the same.
S95, S95-E10, S98, what to choose?
Whether it is the SP95 or the SP98, vehicles put into circulation after July 1990 can drive interchangeably with these two types of gasoline. As explained in the introduction, the visible difference lies in the figure: 95 and 98, representing the octane index. This index assesses self-ignition resistance in a gasoline engine. The higher the index, the greater its strength, its combustion is controlled, thus protecting the performance and longevity of the engine, especially the valves and the injector.
An old car collector could choose to fill his tank with SP98 so as not to risk damaging his engine with a lower octane index. There is no risk, however, for modern vehicles equipped with a rattling sensor to adapt the engine to a lower octane rate or on the contrary to benefit from better performance with a higher index.
For the most curious of you, you’ll have noticed two octane index typologies displayed on gas station pumps: RON (Research Octane Number) and MON (Motor Octane Number). The first indicates the behaviour of a fuel at low speed and during acceleration, the second indicates the behaviour of high-speed and high-load fuel. It is the RON typology that refers to the 95 and 98 on the two unleaded species.
Whether for SP95 or SP98, these two fuels are made of highly toxic solvents, so avoid using them as a cleaning or degreasing solution. Also be careful not to breathe the gases that escape from it even if their smell can sometimes be intoxicating. SP98 appears to be more detergent and corrosive than SP95, especially on rubber parts. The improved version of the SP95, the SP95-E10 (commonly shown E10 on pumps), released in 2009 in France and 2011 in Europe, seems to be more environmentally friendly because it contains up to 10 ethanol, a vegetable alcohol made from beetroot or wheat (E10 – Ethanol 10). For the record, SP95/98 also contains ethanol but only up to 5. As mentioned above, only vehicles put into circulation after 2000 can drive at the E10.
If you have to choose from the three fuels, it’s best to take a look at the gas hatch, check your manufacturer’s manual or simply ask your mechanic for advice. If you find yourself in the situation where you run out of gas and the only fuel available is not the one recommended by the manufacturer, you can always bring a bottle ofoctane booster that will increase the octane index and will act as an injector cleaner, while lubricating the entire injection system.