Non-car enthusiasts are very confused by this question. And the fuel companies haven’t helped – the number of different retail fuel products beggars belief.
In the past few years we’ve seen an explosion in the number of fuel products – many of which bear absolutely no resemblance to the way fuel is actually specified by car manufacturers.
Pop open the fuel flap on a contemporary car and you’re likely to see “Uleaded Fuel Only” inside the flap. Other options include “Premium Unleaded Only” or “98 Octane Unleaded Only”. Imagine how confusing that is when someone in an unfamiliar car pull up at a servo, next to a pump marked ‘V-Power’ or ‘Bio e-Flex’. It can be a difficult code to crack, for the uninitiated. There are consequences - sometimes severe ones - if you get this wrong. You could easily blow up your engine, or cause thousands of dollars worth of damage.
There are currently five different ‘flavours’ of automotive petrols on the market, if you discount the 100-octane leaded avgas sold near some aboriginal communities to discourage recreational sniffing. The widely available mainstream petrol blends are sexed up with a hodge-podge of different brand names for retail sale in the same way, for example, orange juice manufacturers try to differentiate themselves from one another even though it’s really all the same swill, basically. No two fuel brands use the same name for their basic unleaded petrol, even though it is all often produced by exactly the same refinery.
Automotive fuels currently on offer include:
Standard unleaded petrol (ULP), which has an octane rating of 91 and is being phased out in some states, soon to be replaced by E10. It’s the fuel most petrol cars on sale in Australia require. (It’s the one that goes with the ‘Unleaded Fuel Only’ sticker inside the fuel filler flap.)
E10, which is a blend of 10 per cent ethanol. Most modern cars can run E10, as well as many older cars. If unsure, check with the vehicle manufacturer. Older cars might not have E10-proof materials inside the fuel systems. The E10 can, in these cases, break down these incompatible materials and the byproducts of that breakdown can become dislodged and migrate upstream where they can clog the fuel filters and injectors, which is an expensive problem to solve.
If you’ve been using 91 ULP and it’s become unavailable in your area, and your car is incompatible with E10, you’ll have to use the next fuel in our list, a premium unleaded petrol, which is more expensive. (interestingly, E10 can also damage mowers and other yard equipment in the manner described above. It's generally a safer bet to run these items on a premium unleaded fuel. (They consume very little, so the running cost difference is negligible.)
E10 will help a suitably compatible 91-octane-minimum engine perform a little better, too, allowing the engine to advance the timing a little, but it packs less energy into every litre, so your fuel consumption invariably goes up, offsetting most of E10’s upfront price discount. The performance increase will be very minor, and the consumption goes up 3-4 per cent.
Premium, 95-octane unleaded petrol (PULP). This is the first of two ‘flavours’ of PULP, and is the entry-level fuel in some other markets, like Europe, which lacks 91-octane. If your car says ‘Premium Unleaded Only’ inside the filler flap, this is the one you should use – if you use 91-octane or E10, serious, expensive engine damage might result. This 95-octane stuff is the cheaper of the two PULPs.
Premium 98-octane unleaded (PULP). This is the fuel demanded by some exotic cars, often with forced induction (turbo or supercharger induction) or 'direct injection', and the most expensive petrol on the market. If the car says ’98-octane Unleaded Only’ you must not use any of the lesser petrols, because you could severely damage the engine, landing you with a bill an ordinary mortal might not jump over, given the nature of some of the cars that require 98.
E85, a blend of up to 85 per cent ethanol, with the balance in petrol. The recipe varies by place and season; it's not always 85 per cent ethanol. Currently this fuel is available only at some selected Caltex servos and is called ‘Bio e-Flex’. The only cars - so far - that can run it are the latest ‘Flex Fuel’ iterations of Holden’s Commodore. If you stick this green-oriented fuel in basically any other car, chances are you won’t get it started at the next cold start. Cue the expensive repair bill again.
Here’s the thing: you can put a higher octane fuel in a car than the manufacturer’s requirement. 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). A modern engine designed for 91 will even deliver a very small amount of additional power if fed 95 or 98 because it will advance the timing a little more than with 91. The improvement will be very minor. So minor you probably won't notice it.
Most engine design experts I’ve ever spoken to regard running higher octane fuel in an engine designed for a lower octane fuel as basically a waste of money.
However, it’s an unmitigated disaster to put a lower-octane fuel in an engine than the one recommended by the manufacturer. This can lead to severe engine damage. The early detonation of the fuel can raise the temperature inside the combustion chamber to levels that the metal parts inside the engine can't withstand, and unacceptable stresses are also placed on internal components.
One fuel manufacturer that has recently taken a real step in the right direction is Caltex, which has taken the decision recently to display the meaningful numbers – 91, 95, 98 – on every bowser. It's really useful information to help ordinary consumers cut through the branding to the essential information.