Can I directly add 95 gasoline when 98 gasoline is not completely used up?

[DeMicah]

It is possible to add 95 gasoline when 98 gasoline is not completely used up. Occasionally mixing them is acceptable, but it is not recommended to mix them frequently. You should choose between 98 and 95 gasoline based on the suitable gasoline grade for your vehicle. Here are the specific details: Gasoline grades represent the fuel's anti-knock performance. The higher the gasoline grade, the higher the octane number and the better the anti-knock performance. 95 gasoline contains 95% isooctane and 5% n-heptane, while 98 gasoline contains 98% isooctane and 2% n-heptane. Anti-knock performance refers to the ability of gasoline to resist knocking when burning in the engine. It is the primary indicator of gasoline's combustion function. Knocking is caused by abnormal combustion of gasoline in the engine. The octane number is the target for gasoline's anti-knock performance. The higher the octane number, the stronger the anti-knock performance. Higher compression ratios and higher combustion chamber pressures make gasoline with lower anti-knock performance more prone to knocking. Occasionally using the wrong gasoline grade only requires switching back to the correct grade after use. However, long-term use of the wrong gasoline grade can have the following effects: For vehicles that require lower-grade gasoline, using higher-grade gasoline will not cause damage, but the increase in octane number will change the fuel's ignition point, leading to delayed combustion in the engine. This means the engine's power output and thermal efficiency will decrease, resulting in poorer performance. For vehicles that require higher-grade gasoline, using lower-grade gasoline can cause engine knocking. Because the octane number is too low, the fuel's ignition point decreases, causing premature ignition during the compression stroke. If the fuel ignites before the spark plug fires during the compression stroke, resistance will occur during the upward stroke. This resistance can make the engine run very unstable. If the knocking is imperceptible, it only increases noise and does not cause significant damage to the engine. However, if the knocking is noticeable, it indicates severe engine conditions. The vibration not only affects driving stability but also causes abnormal wear on the pistons and cylinders, and in severe cases, can lead to cylinder scoring.

[StKira]

I've experienced a similar situation before - I added 95-octane gasoline directly when there was still quite a bit of 98-octane fuel left in the tank, and it drove without any issues at all. Modern car engines are quite smart; they have sensors and computer control systems that can automatically adjust ignition timing to accommodate differences in octane ratings between gasoline types. The key is to check the minimum recommended octane rating in your owner's manual. If your car normally uses 95-octane, mixing in 98-octane poses little risk. But for cars that recommend 98-octane or higher, it's best not to do this long-term as it might affect acceleration performance or fuel consumption. That said, occasional mixing is safe - I've tried adding the wrong fuel a few times in emergencies without any engine trouble. However, I still recommend maintaining consistency since higher-octane fuel can prevent knocking and make the car run smoother. For daily driving, pay attention to fuel cleanliness and regularly check the fuel tank cap seal to prevent impurities from entering.