What is the right fuel to use in my car?

Australian motorists usually fill the tank with the same fuel, day in, day out… for years on end.

So the range of alternative fuels on offer in this country may have escaped their attention. It's this situation that occasionally leads to a motorist filling a petrol tank with diesel, or vice versa.

How do you know what to use in your car? Usually the owner's manual will spell out what the manufacturer recommends for your vehicle. Look under 'fuel' in the index to find the right page.

Another tell-tale is the filler flap. When opened it may display a warning not to fill using anything less than 95 RON/premium unleaded petrol (PULP). That's the octane rating, which essentially runs from lowest of 91 to premium fuels, the notional highest being 98 RON.

Alternatively there may be advice inside the flap that the car is compatible with (ethanol-blended) E10, or there may be a warning to fill using diesel, if the car is not a petrol-engined model. If there's no warning label inside the filler flap, that most likely means the car is compatible with standard (91 RON octane rating) unleaded.

But there are always exceptions, and older cars are chief among those.

If your car was sold in Australia prior to 1986, it was probably designed to run on leaded, which was phased out here in 2002.

Exceptions include the many older cars that were modified with harder valve seats in the engine's cylinder head to cope with unleaded petrol (ULP).

Also, diesel vehicles sold prior to 1986 are unaffected, since leaded was petrol, as is ULP. Beware of any car that has been converted during its working life. Some taxi operators in New South Wales converted HZ-series Holden Kingswoods to run diesel engines!

Grey imports from Japan and the USA built from 1974 may run acceptably well on ULP, since those markets took the lead-free path 12 years earlier than Australia. Beware of cars that have undergone an engine swap, however, either here or in the home market.

You have a couple of options open to you. Additives dropped in the tank at intervals will keep the engine in top condition, and if you care about engine longevity and performance, use 95 RON PULP. Prior to the introduction of ULP, cars were often powered by engines with higher compression ratios.

The other option is to have a mechanic lift the cylinder heads off the engine and replace the existing valve seats with a hardened type. That reduces the need for an additive to prevent valve seat recession, but you should continue to use 95 RON unleaded petrol.

If you drive a Ferrari or Lamborghini, the manufacturer may specifically recommend 98 RON PULP. This is the highest volume-selling PULP you can buy in Australia. It's possible to obtain higher octane PULP in Australia – up to 110 RON – but suppliers (BP, for instance) specifically recommend against using their 100 RON racing fuel in any road-going passenger vehicle, particularly cars with catalytic converters.

100 RON is intended for off-street competition cars only, and can be expensive to buy.

If you own something like a Volkswagen Golf or a Renault Megane 95 RON (PULP) is the petrol to buy. European markets don't consume 91 RON, so 95 is the default there. The car companies design and tune engines for what we term PULP, not for the cheap and nasty stuff we buy en masse here.

Your car will run on 91 RON petrol, but performance is affected and using the cheaper stuff may have longer-term ramifications. So avoid filling the tank with 91 RON unless there really is no alternative.

There are now high-octane products available for diesels, but unlike high-performance petrol vehicles, diesels will cope with any octane rating, although the high sulphur level that detracts from fuel quality in Australia can impact on particulate filters of recent-model vehicles sold here.

Also, be aware of filling the tank of your passenger vehicle from the high-flow pumps used to fill the massive tanks of long-haul trucks.

If you own an old Falcon, Commodore or Kingswood that was formerly a taxi, it is very likely to run on LPG (Liquefied Petroleum Gas). It may be a dual-fuel set-up too, needing petrol AND gas – the former for cold starting – to run efficiently.

Up until the end of local manufacturing in Australia (October 2017) it was possible to buy new cars designed solely to run on LPG. These were the Holden Commodore monofuel and the Ford Falcon EcoLPI. They only require the gas to run.

It shouldn't need to be spelled out, but just in case... what looks like a filler flap on your Mitsubishi i-MiEV or Nissan LEAF is a flap for an electric recharging port. DO NOT ATTEMPT to pump any liquid in here.

The service adviser or sales person at your local dealer has told you your car can run on E10 or E85 – and that's also stated on the label inside the filler flap. What does that mean though?

E10 and E85 are known as 'blended fuels'. The E stands for 'ethanol', which is an alcohol usually produced from crops. Mixed with petrol it can be a cheaper alternative to premium fuels (PULP) and can lift octane levels slightly. However, it will rot the insides of your car's fuel delivery system if the car is not designed to handle the ethanol content.

Many cars will run safely on either E10 or 91 RON, but E85 (a mix of up to 85 per cent ethanol and 15 per cent petrol) was only ever supported in Australia by Holden. Commodores from late-model VE-series through to the last of the VF-series can run on E85, but the pumps are hard to find these days and the E85 type can be temperamental for starting in cold climates.

There are some plans afoot to introduce hydrogen refuelling capability in Australia. Most vehicle owners and drivers won't need to worry about this for a few years yet. Hydrogen doesn't combust as such; it generates electricity when recombined with oxygen to emit water.

If you own a Toyota Corolla and you've filled the tank with 95 or 98 RON PULP, it's fine. A higher octane product is usually better than lower octane.

If you've filled the tank of your Nissan GT-R with 91 RON, drive the car gently on that tank and then refill with 95 or 98 RON. The modern car's engine-management system will adjust timing to suit the lower octane. You may note impaired performance, but there shouldn't be any lasting effects.

If you've filled your car's diesel tank with petrol (or a petrol tank with diesel), do not start the car. If it's running, stop the car as soon as it is safe to do so. Do not leave the engine running. Call a (mobile) mechanic to drain the tank and all the lines to the engine.

If the government abolishes 91 RON one option may be E10, if that remains available. But the best option for most motorists is likely to be 95 RON PULP.