Can I Add 95 Gasoline Directly Without Using Up 98 Gasoline?

[StJeffrey]

You can add 95 gasoline after using 98 gasoline, but you need to use up the 98 gasoline first before adding 95 gasoline. If the owner wants to change the gasoline grade, they should use up the gasoline in the car's fuel tank until the fuel gauge light comes on before adding another grade, or directly extract it and then switch to another grade of gasoline, rather than mixing them directly. Occasional mixing is not a problem, but frequent mixing is not recommended. Below are the 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 gasoline's ability to resist knocking when burning in the engine. It is the primary indicator of gasoline combustion performance. 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. The higher the compression ratio, the higher the combustion chamber pressure. If gasoline with lower anti-knock performance is used, knocking is more likely to occur. Occasionally adding the wrong gasoline grade is not a problem as long as you 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 designed for lower grades, accidentally 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 designed for higher grades, using lower-grade gasoline can cause engine knocking. Because the octane number is much lower, the gasoline's ignition point decreases, causing it to ignite prematurely during the compression stroke. If the gasoline ignites before the spark plug fires during the compression stroke, resistance will occur during the upward stroke. This resistance will make the engine run very unstably. If the knocking is not noticeable, it will only increase noise and not significantly damage the engine. 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.

[OAutumn]

I've been driving this car for several years and occasionally topped up with 95 octane gasoline before completely emptying the 98 octane fuel in the tank. The car seems to run fine without major issues. Modern engines are intelligently designed to adapt to gasoline with different octane ratings. Since the difference between 98 and 95 octane isn't substantial, the engine can still operate normally when they're mixed, without noticeable knocking or stalling. However, the engine's power output might be slightly weaker after mixing fuels, and throttle response may not be as crisp as when using pure higher-octane fuel. If it were me, I'd recommend either using up the 98 octane before switching to 95, or waiting until the tank is nearly empty to change grades, to avoid long-term fuel mixing that might affect spark plug or sensor lifespan. This isn't a major problem, but accumulated minor issues could impact fuel efficiency and maintenance costs. Next time before switching grades, just check your vehicle manual to confirm the recommended octane rating.