It can't hurt your engine if you use a higher octane fuel. So, for example, if you use 95 or 98 in an engine designed for 91, that's OK. However, it's not acceptable to put in a lower octane fuel than the minimum recommended by the manufacturer. Using 91 in an engine designed for 95 or 98 is potentially destructive.
There is no risk when putting higher-octane fuel in your petrol car.
Mixing the 95 and 98 octane fuels will not cause any problems.
The differences between 95 RON and 98 RON
Fuels such as 95 or 98 RON (octane rating number) have a higher resistance to burn which indicates higher levels of energy available for the vehicle's engine.
No problem with putting 95 or even 98 in a car designed for 91 – except generally you will be wasting money. Higher octane fuels don't contain more energy. Octane rating is basically resistance to burning under pressure, allowing higher compression ratios to be used (cylinder pressures, actually).
Depending on the car you own, fuelling up with 98 octane can result in increased performance that will see you produce a little extra power when compared to lower octane petrol. But if you're driving something older that hasn't been tuned to run on high-octane, you'll likely see no tangible benefit.
Suzuki SA, advises that 95 octane fuel may not offer a significant boost in performance but it does offer an advantage in keeping your engine clean of carbon deposits, and this could help extend the lifespan of your car (if you follow the recommended maintenance guidelines).
The higher octane gives premium gas greater resistance to early fuel ignition, which can result in potential damage, sometimes accompanied by audible engine knocking or pinging.
Premium, 95-octane unleaded petrol
Premium unleaded 95 petrol (PULP 95) is a type of unleaded fuel that is designed to be more fuel efficient and cause a smoother engine operation, which in turn improves performance. It is meant for use by imported and high-performance vehicles, but can be used by the majority of cars.
The difference between each fuel type is their 'octane rating', or the numbers that identify them, such as 91, 95 or 98. The octane rating is an index of a fuel's heat resistance, which means the fuel's ability to resist premature burning, which can cause your engine to 'knock'.
The gas will mix in the tank and become something like 94 octane. Your car won't know the difference. If they are mixed in equal parts, you have 96.5 octane.
Premium gasoline costs a lot more, but, although it doesn't have ethanol and might have a few added detergents, it's not better. Mixing the grades to get 91 or so shouldn't hurt, but it probably isn't saving you much money.
If you keep using lower-octane fuel for a prolonged period, you will have a damaged engine and emission control system. But if you use a higher-octane gas than your car needs, it won't do any harm, nor will it benefit your car.
If your vehicle manufacturer stipulates the use 95RON or 98RON, then yes, it's worth the extra cost. If your car owner's manual says that your car doesn't necessarily need premium but will run a lot better on higher octane fuel, then obviously it's your call.
If you're unsure of your car's minimum octane rating, it should say on the inside of the fuel flap. If it says “unleaded petrol only” it means 91 octane fuel is OK. If the fuel flap says “premium unleaded only”, it means you need to use at least 95. If the fuel flap tells you to use 98, that's what you need to do.
Can I mix premium unleaded with super unleaded? If you mix premium unleaded (which has an octane of 95 RON) and super unleaded (97/98 RON) evenly in your tank, you'll end up with a mixed-grade petrol of around 96 RON.
A premium unleaded fuel like Shell V-Power 98 has been infused with powerful cleaning agents that remove gunk (aka carbon deposits, if you want to use the technical term) from your fuel injectors and inlet valves, and it also protects against corrosion, which can lead to serious problems for your car's innards.
Will higher octane gasoline clean your engine better? No. High octane gasoline doesn't outperform regular octane in preventing engine deposits from forming, in removing them, or in cleaning your car's engine. This is a myth.
Using different octane is not going to cause severe damage, but things can get tricky when you use a lower one. Depending on the model of your auto, the lower octane might work fine, but it could also cause damage. You will be able to notice poor engine performance and even overheating.
Other things to consider: failing to use high-octane fuel in your premium vehicle could cause engine knock or pinging, which sounds like there's a large woodpecker under your hood; you could experience damage to your components; and using the wrong octant level could even void your warranty.
In a vehicle that is recommended to use regular unleaded (91) fuel - you can opt for premium 95 or 98 unleaded fuel. This will not cause any harm to your engine.
Octane is a measure of how much heat and pressure a fuel can withstand before igniting. That's why higher performance cars with their high compression ratios require the higher octane fuel such as 95 and even 98. The inside of a fuel flap usually displays the recommended fuel octane rating.
Octane ratings are plastered all over every gas pump, and, contrary to marketing terminology, "premium" does not necessarily indicate a fuel is "better" for your car as much as whether it is "right" for your car. Fuel with an 87 octane rating burns more quickly while higher-octane fuels burn more slowly.
A car will always perform better on the recommendation from the manufacturer. However, you can use fuel the other way round – putting 95-octane in your car if you car usually runs on 91. It won't do too much in terms of the performance of the car, and it may not always be worth the extra cost!