Dispelling misconceptions: how to accurately interpret engine oil condition

One widespread concern among drivers is the black colour of engine oil in a diesel engine, even shortly after it has been replaced. This could induce worry as it appears as though the oil hasn't been renewed. Naturally, we're addressing engines that have accrued some mileage.

However, specialists from TotalEnergies reassure that this occurrence should be a source of relief. The black colour of the oil indicates that it has absorbed soot and binds it, preventing the accumulation of deposits in various engine components. Provided the oil was appropriately selected for the engine, soot within the oil doesn't impair its properties — this is the way it's meant to function.

An unexpected sight for some drivers is extremely clear oil. It's so transparent that it's hard to spot. Such oil is often found on the dipsticks of new cars freshly out of the dealership. To check its level, you need to angle the light just right. But what about older cars?

Clear oil is mostly seen in engines fuelled by autogas. LPG promotes cleaner burning and doesn't lead to the creation of large amounts of pollutants, which means the oil retains a fresh-like colour for longer. Should this thrill drivers? Absolutely, however, they should not rely mostly on the oil's colour when deciding to replace it.

In oil that has worked a bit, yet remains clean, invisible chemical impurities start to accumulate. These contaminants gradually degrade the oil's lubricating properties. This primarily concerns sulfur, which is found in LPG.

Sulfur leads to the oil becoming acidic, deteriorating its lubricating properties. This is why some manufacturers provide oils with a composition that prolongs its lifespan. In the case of cars fuelled by LPG, experts from Total recommend more frequent oil changes than in cars running on other fuels, if standard oil is used.

Another characteristic drivers pay attention to is the viscosity of the oil, which allows them to gauge its density. For many people, it's straightforward that 0W-20 oil is thin, and 10W-60 oil is thick. These terms might be used casually, but in a practical sense, it can vary significantly.

It all hinges on the conditions in which the oil operates, and its properties. Low-viscosity oil may become thick at elevated temperatures, while high-viscosity oil may thin out during engine operation. This is influenced by the oil's viscosity index.

For instance, 15W-40 oil, in comparison to 0W-40 oil, will possess a higher viscosity at room temperature. Nonetheless, at a temperature of 100 degrees Celsius, both oils will have identical viscosity, and at higher temperatures, i.e., in the engine's hottest parts, 0W-40 oil will have a higher viscosity than 15W-40.

Experts from Total advise that seeking oil with the highest possible viscosity is extremely risky, as the engine manufacturer's standards are paramount. Either too low or too high oil viscosity can impact engine operation resistance and the performance of hydraulic components, such as valve lifters, variable valve timing, or tensioners.

The result of imprudently changing your oil can lead to an increase in engine noise, reduced performance, and in drastic situations — failures, such as an extended timing chain.

It is crucial, even if selecting the viscosity of the oil yourself, only to use the range acceptable to the engine manufacturer, information which can always be found in the instruction manual.

Andrzej Husiatyński, an expert from TotalEnergies, concludes, "modern engine oils are technically so complex that they require buyers to peruse labels meticulously. It's not sufficient just to know the oil's viscosity, as diverse oil variants exist within one category, differing, for instance, by manufacturer's approvals. We mustn't solely depend on the oil's appearance in the packaging, especially since we typically observe it at room temperature. The colour, transparency, and smell of oil during operation don't impart any information about its lubricating properties and can be misleading."