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What Petrol Should I Use?

When you pull up to the bowser at your local fuel station, you’ll see a variety of pumps with different labels. You probably know which fuel is meant to go in your car, but have you ever wondered what the differences are and what happens if you use the wrong fuel?

With decades of experience in the mechanical industry, the team at Donnellans are able to help you out with all of your fuel related questions.

If you have a diesel vehicle, keep an eye out for our upcoming article about different ways to charge electric vehicles and another about the different grades of diesel fuel available in Australia (spoiler alert: at the time of writing, there’s only one grade of diesel in Australia).

Petrol is made from petroleum, itself a refined product made from crude oil, which is pulled from the ground by tapping into massive underground reservoirs all over the world.

All petrol designed for use in Australia is unleaded, since added tetraethyllead was phased out in 2002 due to health concerns regarding lead in exhaust emissions. Lead was originally added as a cleaning agent and lubricating additive which helped avoid engine inlet and exhaust valves being damaged over time. Lead also helped with avoiding knocking or pre-detonation.

Leaded fuel is still widely used in aviation, so if you live near a light airport, you’re probably still being exposed to trace amounts of lead through your environment.

What Do the Numbers on Fuel Pumps Mean?

The numbers such as unleaded 91, unleaded 95 and unleaded 98 refer to Research Octane Number, commonly called RON, which measures resistance to engine knock or pre detonation.

Higher RON numbers mean that the fuel is more resistant to pre-detonation, or ‘knocking’. Many modern engines can detect knock and change timing or fuel delivery to avoid engine damage.

Unleaded 95 explained

Unleaded 95 is the default grade for most modern petrol cars and is often advised by manufacturers for everyday driving. It typically includes stronger detergent packages and tighter chemical specifications that help reduce injector and intake valve deposits. If you want a model-specific check, book a basic servicing appointment with our team at Donnellans. We will confirm your vehicle manufacturer's guidance and inspect the fuel system.

Unleaded 98 explained

Unleaded 98 is ultra premium unleaded fuel and benefits high performance engines with higher compression or forced induction. Vehicles that specify 98 are engineered to extract measurable performance and smoother operation when supplied with that higher octane. For standard vehicles that call for 95, routinely using 98 rarely gives enough benefit to justify the extra cost unless the engine has been retuned by a reputable tuner.

E10 and E85 ethanol blends explained

E10 is unleaded petrol blended with up to ten per cent ethanol and remains an unleaded fuel. E10 has slightly lower energy density than pure petrol so that you can expect a modest reduction in kilometres per litre. Most modern petrol cars sold in recent decades are E10 compatible, but you should confirm compatibility in your owner's manual. For small engines, classic cars and some motorcycles, ethanol can corrode seals or attract moisture, so avoid E10 for equipment that is not E10 compatible.

If you need a compatibility check, bring your vehicle to Donnellans for a targeted inspection as part of a logbook servicing appointment. E85 is a high ethanol blend used mainly in motorsport and flex fuel vehicles. It provides very high knock resistance for aggressive tuning but it reduces range considerably. Only use E85 in vehicles designed or converted for flex fuel operation and with fuel system upgrades.

Additives, detergents and deposit control

Modern unleaded fuels contain detergents that prevent injector and intake valve deposits. Deposits reduce economy and increase emissions over time. If your car shows rough idling, hesitation or loss of economy, the issue may be deposits rather than fuel grade. A professional fuel system clean and injector service can restore performance. Our basic servicing and log book servicing include checks that identify deposit related problems and recommend cleaning or repairs where required.

Small engine and classic vehicle considerations

Small engines, classic motorcycles and older cars were often not designed for ethanol blends. Ethanol can corrode older fuel system components or degrade seals and rubber parts. For lawn mowers and older machines consider ethanol free options where legally and practically available, or consult a qualified mechanic. If you have a classic car and you are unsure, book a consultation so we can advise whether fuel system upgrades are needed.

Warranty and manufacturer guidance

Using the fuel grade specified in the owner's manual helps protect warranty coverage. If a manufacturer explicitly advises against a fuel type and you use it, warranty claims related to fuel or emissions hardware may be affected. Keep fuel receipts and service records, and bring the vehicle to a qualified centre if you have fuel-related symptoms.

Misfuelling, emergency steps and what not to do

If you accidentally put unleaded petrol into a diesel vehicle, do not start the engine. This is a high-risk error that requires the vehicle to be isolated and professionally drained and flushed. If you put diesel into a petrol car, do not run the engine extensively. Diesel in a petrol engine causes heavy smoke and poor running and will require cleaning and inspection. Contact a qualified mechanic promptly and, if needed, book a diagnostic so the vehicle can be checked and any contaminated fuel removed.

Does It Matter Which Fuel You Use?

There’s a quick and easy answer to this question. Yes.

Generally you can put a higher grade fuel in your vehicle without worry, but it’s highly recommended not to run a spec fuel that’s lower than what your manufacturer has stated.

While many modern vehicles will adapt to a lower rated fuel at the cost of performance and economy, you risk pre-detonation and potential engine damage over time by using the wrong fuel.

In many cases, running a higher grade fuel that what your vehicle was designed to use won’t net you any benefits unless your vehicle is modified or re-tuned to take advantage of the higher grade fuel.

You can find your manufacturer's recommended fuel grade in your owner's manual, or by contacting Donnellans, we can let you know all sorts of information regarding your vehicle.

FAQs

What does unleaded mean?

Unleaded petrol is fuel without lead additives. Australia phased out leaded petrol for road vehicles in the early 2000s, which improved public health and enabled modern emissions technology to function correctly.

Is unleaded petrol the same as unleaded gasoline?

Yes the terms unleaded petrol and unleaded gasoline refer to the same product. Use the local term that best fits your audience.

Is E10 safe for my car?

Most modern petrol cars are E10 compatible but check the owner manual or ask a qualified mechanic. For small engines and classic vehicles do not assume compatibility.

Will switching to unleaded 98 improve my fuel economy?

For most drivers, upgrading to unleaded 98 when the car requires unleaded 95 will not improve fuel economy or power unless the engine management has been retuned to exploit higher octane.