Slightly off topic...But has anyone actually tested to see if the fuel from Gull or BP is actually dirty or more dirty than that from Caltex, Mobile or Zed Stations? Or is the assumtion based on the often lower price?
Gull have admitted to adding butane in the past to their fuels.
Gull have been sneaky about putting biofuel in the mix in the past without a large warning on the pump and signs that their fuel isn't suitable for most cars.
Only good thing gull does is make other nearby gas stations drop their prices to compete with gulls fuel.
If it runs fine on 91, a higher octane fuel will not hurt it, but you may not be getting the best fuel economy possible.
The octane rating of fuel indicates the fuel's resistance to 'knock' (That is ignite earlier than intended due to high compression). As others have pointed out, most modern cars have pretty sophisticated computers that will detect knock and adjust timing to prevent it. The result of that will be a decrease in power and fuel economy.
The best thing for you to do is to try out a higher octane fuel. 95 will be sufficient (I guarantee now that 98 RON will be a waste of money in a 1500cc Mazda). Run a couple of tanks and see if your fuel economy improves, then work out if it's worth the extra money.
There is a lot of misconception about what octane ratings in fuel really means. Many people seem to think that higher octane = more energy in the fuel, and that higher octane fuels are 'cleaner' and lower octane fuels are 'dirtier. All of which are totally untrue, and simply oil company FUD.
Higher octane fuels actually have a slightly lower amount of energy in the fuel, and this is because there are additives in the fuel that increase the RON, and therefore increase the resistance to knock. Basically, they are watering down the higher octane fuels do it doesn't ignite as easily. So when you hear the oil companies tell you that your car will run better on higher octane fuel, that is only true if it was designed (or modified) for a higher octane fuel, but you're currently running a lower RON fuel and the car is adjusting for knock. If your car is tuned for 91, putting a higher octane fuel in the tank will do nothing other than drain your wallet a bit faster. The same is true for putting a lower octane fuel in a car designed for a higher RON - your car will adjust itself to the fuel, at a cost of fuel economy and power (and therefore dollars out of your wallet.
Lower octane fuel is not 'dirtier' either. This misconception largely comes from the 'premium' label given to some higher octane fuels that have detergents added to them that supposedly 'help clean your engine'. Firstly, AFAIK there is no reliable evidence to actually prove that these detergents actually do make a difference, and some suggestions they could even be harmful to your engine. So you're paying a price premium for fuel that really gives you no benefit.
Also a note for anyone who feels the need to use ethanol blended fuels: while these are cheaper at the pump, you will almost certainly obtain poorer fuel economy from the vehicle to the point where you fuel cost per kilometre will actually be higher than using regular 91. (A small number of exceptions to this for cars specifically tuned for it). Ethanol doesn't burn as well as petrol, so as a fuel it's less efficient.
Ethanol blended fuels are, for the most part, another fuel company marketing tool to make you think you're getting a better deal, but actually getting more money out of you.
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long answer is whatever the manual says, which is probably 91. even if you go against the grain and use 95 you might gain a bit in economy (or not) but waste money on the fuel cost which evens out, so use 91. if you think 95 is cleaner than 91 well even if that's true your car is now 7 years old and using a cleaner fuel won't undo all the past muck. even if it's cleaner how much cleaner? 0.1%? 10%? 100%? the most important thing is change oil and filter and it will run longer than you want to keep it for.
but if it is a new car and 95 is 100% cleaner i'd use 95 ... but it's probably not so i'd use 91. (does anyone know how dirty is 91 vs the premium fuel?)
DravidDavid: Slightly off topic...But has anyone actually tested to see if the fuel from Gull or BP is actually dirty or more dirty than that from Caltex, Mobile or Zed Stations? Or is the assumtion based on the often lower price?
I fill up whereever the fuel is.
Gull are the only ones with a separate distribution network, all the others get their fuel from the Marsden Point refinery, either directly or via Wiri. They fill up their own tankers from the same source, then chuck in a couple of teaspoons per truckload of their "secret ingredient" before delivering to their stations. So, other then the quality of the on-site storage tanks at each service station, there is no difference in quality between BP, Caltex, Mobil or Zed because its the exact same fuel.
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