The Latest

Myth Buster: Busting the myths on diesel fuel

Diesel is the undisputed lifeblood of commercial land transport that keeps the economic wheels rolling in most countries, yet it’s often touted as the ‘bad boy’ when it comes to pollution. We’ve compiled a five-part Myth Buster blog series to dispel some of the most common untruths surrounding this cost-effective fuel type.


In this first delivery, we’ll look at the common perception that diesel is dirty, that colour affects performance, that all diesel is the same, and that density impacts quality.

Myth: Diesel is a dirty fuel

Most technologies, including diesel technology, goes through iterations of continuous improvement. The refining sector is seeing substantial investment to reduce sulphur content, thereby lowering greenhouse gas emissions while attempting to maintain diesel fuel’s low cost. Notably, low sulphur diesel has greatly reduced sulphur dioxide and particulate matter emissions, both of which play a major role in air pollution.
Although diesel is a heavier fuel than petrol, it burns very cleanly provided the engine is in good condition.

To help keep engines in such a condition, our diesel with the Techron® D additive helps keep expensive fuel injectors clean, protect against corrosion, and reduces foaming while filling up. Don’t be fooled by the price either, as cheaper isn’t always better. If fleet owners want to maximise their vehicles’ longevity, they should prioritise quality first, especially given the high cost of long-haul diesel truck engines.


Modern fuel systems are designed to reduce emissions and fuel consumption and improve engine performance, but they can only achieve this at very high pressures. However, a precondition is that the fuel used must be clean. For fuel to be classified as ‘clean’, it must meet a rating of 18/16/13 on the ISO 4406 cleanliness code. Many of our customers and OEMs request this level of cleanliness.


Myth: The darker the colour, the better the performance

Similar to the misperception that diesel is a dirty fuel, there’s a commonly held perception to equate diesel colour with performance – the darker, the better, as some would say. As a result, some believe that if the product is unusually pale, there must be an issue with the quality. This perception is more prevalent in the commercial and industrial sector because, unlike retail, they are more exposed to fuel theft and product adulteration.

However, the colour of fresh, uncontaminated diesel has no influence at all on its quality or fitness-for-purpose. Depending on the method of manufacture and the sulphur content, diesel can be straw-yellow to water white.

Grades with a low sulphur content (10mg/kg) have an inherently lower density and are generally paler. This is because the colour bodies in the fuel diminish during the hydrodesulphurisation process. Over time, particularly during long-term storage, diesel starts to take on a darker, amber colour – caused by a reaction to atmospheric oxygen. This results in a rise in the acidity level and if stored too long, may even form insoluble oxidation products referred to as ‘gums’.

The reasons for prolonged storage may vary, such as storing diesel for strategic inventory or emergency electricity generation. At Caltex, we have a range of antioxidant additives to counter the effects of long-term storage.


Myth: All diesel is the same

Just as petrol is rated by its octane, diesel is rated by its cetane, which indicates how easy it is to ignite and how fast it burns, so diesel fuels are not equal. Standard diesel fuel in the SA market comes in two grades – 50ppms and 500ppms (parts per million Sulphur) – of which 50ppmS holds around 80 percent market share. The former helps extend engine life, meaning machinery can operate better for longer – a massive benefit in high-cost industries such as mining or farming.

Viscosity also plays a significant part. The greater the viscosity, the greater pressure or load it can withstand, and the better it maintains separation between moving metal parts. But there are limits. If the viscosity is too high, it won’t flow as readily, and your engine will work harder and burn more fuel, whereas too low viscosity may lead to a lack of lubrication.

Like any oil, diesel gets thicker and cloudier at lower temperatures, even turning into a gel in extreme conditions. This is a rare occurrence, though, as the fuel must stay very cold for long periods before it will gel. The temperature at which the naturally present paraffin wax in diesel begins to crystallise is known as the cloud point, as the solidified wax gives the diesel a cloudy appearance. However, all road transport diesel fuels are designed for better lubricity and an improved ability to handle winter temperatures. These fuels contain very little paraffin, and therefore has a significantly lower cloud point.

There are also different levels of additisation of diesel products in the market, with varying levels of detergents that help keep injectors clean. For instance, the addition of performance additives such as Techron D® cleans your fuel system and improves fuel economy, helping to meet environmental and fuel efficiency standards.

Myth: Denser diesel equals better diesel

As mentioned earlier, diesel grades with a low sulphur content are inherently lower in density. However, since they are still within specification, the lower density will not impact its quality or performance.
Diesel density ranges from 800kg/m3 to 860kg/m3 and is 15-20 percent higher than the density of petrol. Yet, while their density specifications don’t overlap, the two can mix if there is cross-contamination.
It’s therefore useful to know the original density of a batch of fuel being delivered. If the density of the fuel as received is significantly different from its original density measurement, it indicates possible contamination. Consequently, diesel will never be transported with black oil products that can cause contamination.

In Need Of Technical Advice? Get In Touch With Our Specialists Today!
Unmatched work capability

Continued progress on energy efficiency, lower emissions, and growing interest in renewable low-carbon biofuels is living proof of diesel’s expanding capabilities in our sustainable future. And with ongoing diesel technology improvements, the commercial sector will continue to benefit from diesel’s unmatched work capability as the best option for fleets and businesses.