Is premium diesel worth it?

And the answer is a little bit complicated
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It’s a regular sight on the forecourt these days – premium diesel. While it might sound like premium unleaded, with its 95- or 98-octane rating, you need to dig into some extra details to know about it.

Costing a little more each time you fill up, is premium diesel worth the outlay?

Whereas premium petrol has a higher octane rating over the regular stuff, which can bring benefits of greater power outputs for some vehicles, the benefits of premium diesel are more nuanced.

Octane ratings for petrol refer to a fuel's resistance to detonation (early ignition). A higher octane is effectively a better-quality fuel, which some vehicles need to run properly.

Diesel fuel gets a cetane rating instead, which relates to the speed with which the fuel burns.

Is premium diesel better quality with a higher cetane rating, then? No. It’s the same as regular Joe diesel, but with a different additives package being the differentiator. Otherwise, it sticks to the same ultra-low sulphur rules and standards as regular old diesel.

Since 2009, all of Australia's diesel is required to be ultra-low sulphur, which has 10 parts per million (PPM) sulphur content. The standards also dictate things like cetane ratings, lubricity, biodiesel content and cloud point (more on that later).

Vortex Diesel is Caltex’s brand of premium diesel. It carries a cleaning agent, rust inhibitor and foam inhibitor in comparison to normal diesel fuel. Exact details of the ingredients and benefits aren’t on its website, and even though it quotes an industry-standard 32-hour testing procedure, Caltex falls short of any specific claims.

Instead, the find says it was able to restore a ‘significant’ loss of power. How significant? That remains undisclosed.

Shell’s fine print sums it up well, with each claim of its own V-Power diesel on its website followed by this phrase: “Actual effects and benefits may vary according to vehicle type, vehicle condition and driving style”.

If you’ve got an old diesel motor, with a handful of runs around the sun and plenty of digits on the odometer, then there is perhaps merit to running a tank of premium diesel from time to time for a few extra cents per litre. The additional detergents in particular could help clean some gunk in your fuel system for smoother and better running. However, any helpful evidence is more anecdotal and based on the fuel company’s claims, rather than fact or independent verification.

There might be more benefit in saving your money, and spending the few dollars on some kind of additive that cleans injectors. But once again, all of these products aren’t necessarily proven to work, so you’ll need to exercise some faith in their claims to fully believe them.

However, if your diesel-powered vehicle is newer and in good mechanical condition, there isn’t a lot to gain from the extra expense, however small it might be. Using the premium stuff might help keep your new fuel system otherwise clean, but it's something virtually impossible to quantify.

What about truck diesel?

Don’t think this is some kind of brawny or heavy-duty diesel that will put hairs on your chest. In fact, trying this out might make a horrendous mess.

Truck diesel, often set aside at the service station, uses a high-flowing diesel bowser with a commensurately big nozzle. This saves time if your fuel tank measures in the hundreds of litres, but the nozzle will not fit passenger cars and four-wheel drives. And if it does fit, the flow might overcome the filler and breather pipes and spurt out everywhere. Ask me how I know.

Alpine Diesel and Highland Diesel

In cold temperatures, regular diesel fuel will start to solidify or ‘wax up’. Naturally, this poses a few problems for your engine.

Fuel manufacturers refer to something as a ‘cloud point’, which refers to the temperature at which the fuel starts to gain a cloudy appearance as it crystallises and solidifies.

While fuel suppliers will vary the cloud point according to the seasons, particularly cold areas like Tasmania and the so-called Australian Alps require an even lower cloud point.

So, while a LandCruiser Prado could potentially drive all the way from Birdsville to Cabramurra on one tank of fuel (just), it’s perhaps not the best idea. Fuel up closer to your cold-climate destination to ensure you don’t wake up to a two-tonne paperweight in the morning.