Is it problematic to add 98 octane gasoline when 95 octane is not fully used up?
3 Answers
95 octane and 98 octane gasoline should not be mixed. If you occasionally add the wrong gasoline grade, simply switch back to the correct grade after using it up. However, long-term use of the wrong gasoline grade can have the following effects: For vehicles requiring lower octane, accidentally using higher octane gasoline will not cause damage, but the increase in octane rating will alter the fuel's ignition point, leading to delayed combustion in the engine. This means both the engine's power output and thermal efficiency will decrease, resulting in poorer performance as experienced by the driver. For vehicles requiring higher octane, using lower octane gasoline can cause engine knocking. Because the octane rating is too low, the reduced ignition point may cause premature ignition during the compression stroke. If combustion occurs before the spark plug fires, resistance will develop during the upward stroke. This resistance makes the engine run very unevenly. If the knocking is imperceptible, it only increases noise without significant engine damage. However, noticeable knocking indicates severe engine conditions, affecting not only driving stability but also causing abnormal wear on pistons and cylinders, and in severe cases, cylinder scoring. Besides checking the appropriate gasoline grade in the vehicle's manual, you can also find it on the fuel cap. Typically, the gasoline grade can also be determined by the engine's compression ratio. Vehicles with a compression ratio between 8.6-9.9 should use 92 octane, while those between 10.0-11.5 should use 95 octane. However, with the use of new technologies, the compression ratio alone cannot determine the required gasoline grade. High compression ratio engines can be tuned to use lower octane gasoline because other factors, such as ignition timing, turbocharging technology, and Atkinson cycle technology, also play a role. Generally, the higher the gasoline grade, the higher the octane rating and the better the anti-knock performance. 92 octane gasoline contains 92% isooctane and 8% n-heptane, while 95 octane gasoline contains 95% isooctane and 5% n-heptane.
I have to say, there's usually no big issue with adding 98-octane gasoline when there's still 95-octane left in the tank. I often do this myself, especially when in a hurry or when only higher-grade fuel is available at the gas station—the octane levels will average out after mixing, potentially making the car run slightly smoother without harming the engine. The key is not to switch back and forth frequently, as the car's computer can adapt to minor changes, but long-term fluctuations may lead to incomplete combustion or carbon buildup. My advice is to consider the remaining fuel level: if the tank is only a quarter full, adding 98-octane won't make much difference; if it's more than half full, mixing might be a waste of money, given that 98-octane is significantly more expensive. Remember to regularly check fuel quality to avoid water or impurities getting in, which could cause knocking. In short, it's fine for occasional emergencies, but for long-term peace of mind, sticking to the manufacturer's recommended fuel grade is best.
From a technical perspective, mixing 95 and 98 octane gasoline is generally safe. The key factor is the octane rating difference: 98 octane has superior anti-knock properties. When blended into a lower-grade fuel tank, the resulting octane level falls between the two, preventing engine damage. I've witnessed numerous cases during repairs—for instance, owners temporarily switching to higher-octane fuel before highway trips often report reduced knocking sounds. However, note this: vehicles designed for 95 octane may experience slight hesitation if suddenly filled with excessive 98 octane, as the ECU adjusts air intake more slowly. Optimal practice is to refuel when tank levels drop below 30%, ensuring controllable blending ratios. Compatibility isn't a concern either—modern gasoline additives share similar formulations, posing no corrosion risks when mixed. The real issue lies in developing bad habits that waste money and compromise fuel efficiency.
