Does petrol and diesel quality vary between fuel retailers?

20 October 2023 By: Vanessa Pogorelic

Is the fuel at some service stations better than others? We separate the myths from the facts about fuel quality for WA motorists.

Unlike many of the other branded products we buy, petrol and diesel ‘brands’ are largely a label rather than an indicator of a product with certain unique characteristics.

At least that is the reality in Australia. However, marketing by major fuel companies has led many vehicle owners to believe that one fuel brand may have significantly different characteristics from other competing brands.

RELATED: Fuel myths and facts

So let’s look at the facts and finally settle the debate.

Australia has quality standards that all fuels must meet

In Australia, the Federal Government regulates the quality of all petrol and diesel sold here.

According to the Fuel Quality Standards Act 2000, all petrol and diesel sold in Australia must meet a strict set of operability and environmental standards, with those standards defined for all petrol and diesel fuels sold.

In addition to this, most of the fuel sold in WA is originally supplied by the same source. It’s only after the base fuel is sold that individual fuel companies pour in their additives to certain premium fuels which they claim clean engines.

To ensure fuel standards are adhered to, the National Measurement Institute and the Department of Climate Change, Energy the Environment and Water regularly monitor fuel quality and the accuracy of its dispensing equipment at sites across Australia.

In terms of sulfur content, octane ratings and all the other regulated specifications outlined in the Act, fuel retailers are selling fuel of largely equal quality, because they are regulated to do so.

What about premium petrol?

One measure of the quality of unleaded petrol is its Research Octane Number (RON), which is an indication the fuel’s resistance to detonate when compressed. Such as when it’s inside an engine. The better the fuel’s ability to resist detonation when under pressure, the better its burn can be controlled by the car’s ignition system.

If you use a fuel with a lower RON number than recommended for your vehicle, you may hear a ‘knocking’ sound coming from your engine due to the fuel detonating rather than burning in a controlled manner.

This reduces economy and performance and in time will damage your engine. Premium unleaded petrol carries a RON rating of 95 and 98.

There is a belief that using a fuel with a higher RON number than recommended for your vehicle will enhance its performance and economy. While using fuel with a higher RON number – such 95 or 98 RON in a vehicle recommended to run on 91 - will cause no harm and may deliver very small increases in performance or economy, the additional cost to you will usually outweigh any slight economy benefits.

So save some money at the pump and stick with the fuel recommended by the manufacturer for your vehicle. Using the fuel recommended for your vehicle will, in most cases,provide the most cost-efficient fuel consumption.

However, if your vehicle manual specifies 95 or 98 octane, then you must use these premium fuels.

The amount of 91 RON petrol sold by fuel retailers in WA has been steadily decreasing. This has been driven largely by an increase in the number of passenger vehicles that require the costlier 95 and 98 RON fuels.

The other potential contributor to this increase in the volumes of 98 RON fuels sold is the extensive marketing campaigns that suggest these fuels are better for your car, which leads us to the question of petrol additives.

What about premium diesel?

Diesel and petrol are part of the same crude oil refining process, with diesel being produced at a different stage from petrol in the distillation of the crude oil feedstock.

As with petrol, there are standards that define its quality. In Australia, the diesel fuel standard describes automotive diesel, renewable diesel and synthetic diesel.

All diesel fuel sold in Australia must have a minimum cetane rating of 46. The cetane number is a measure of the speed of combustion.

So what’s the difference between premium and non-premium diesel? The simple answer is – there is very little difference.

Premium diesel fuels have the same cetane number as regular diesel, so there is no performance advantage in using this fuel over regular diesel.

Although premium diesel fuels are sometimes claimed to contain additives designed to clean an engine, the actual benefits of these are unclear and we have not seen any credible evidence of this.

In early 2018, RAC undertook independent laboratory testing of diesel supplied from a number of different branded outlets across the Perth metropolitan area. The results of this testing showed all diesels tested were within the specifications required by the Fuel Standards regulation.

One small difference is that premium diesel is sometimes claimed to have a lower tendency to foam when being pumped into a vehicle's tank, however this does not affect performance.

Of course, the other key difference is that premium diesel fuels are typically more expensive.

For these reasons, we would recommend saving your money and just using regular diesel.

While it’s up to you to judge whether it’s worth paying more for a premium-branded diesel, where that diesel is more expensive, ultimately we recommend buying the cheapest diesel you can find.

Can I use truck diesel?

Truck diesel is identical to regular diesel. The only difference is that the nozzle on the pump is wider and the rate at which it flows from the pump is higher.

Regular diesel also generally costs more than truck diesel. The price difference between the two is largely due to fuel companies’ marketing regular diesel as ‘premium’ fuels with their own additives included.

Unless you are filling a truck, however, you should avoid using the truck diesel pumps at fuel stations due to the high rate of fuel flow.