Popular myths about oils. Does black oil mean engine trouble?

Some myths about engine oils are so common that many drivers judge the condition of the oil based on its color, especially when purchasing a used car. Some even go a step further by touching the oil to check its consistency. Are such actions justified? We debunk the most common myths about engine oils.

One of the most common surprises for drivers is the fact that engine oil in a Diesel engine turns black almost immediately after being changed. This can cause concern, as it may appear that the oil was not replaced. However, this is a normal occurrence, especially in engines that have already done some work.

Experts from TotalEnergies explain that black oil color is a reason to rejoice, not worry. This means that the oil has effectively absorbed the soot and bound it, preventing the accumulation of residues in various parts of the engine. The soot in the oil does not negatively affect its properties, as long as the oil has been suitably selected for the engine — this is precisely one of its functions.

Pure oil, which is almost invisible, can be seen on the dipsticks of new cars. Cars that have just left the dealership have oil so clean that you need the right angle of light to see its level. But what about older cars?

The cleanest oil can often be seen in engines powered by autogas. LPG promotes cleaner combustion and does not produce a large amount of pollutants, which makes the oil maintain a color similar to fresh for a long time. Should this please drivers? Of course, but they should not be guided by the color of the oil when determining the replacement date.

In an overworked but still clean engine oil, invisible chemical impurities accumulate, which negatively affect the oil and impair its lubricating properties. This mainly concerns sulfur, which is present in LPG.

Sulfur causes oil acidification and deterioration of its lubricating properties. That's why some manufacturers offer oils with a special composition, which extends their lifetimes. In the case of LPG-powered cars, experts from Total recommend more frequent oil changes than in non-gas cars, if standard oil is used.

Another aspect often considered when evaluating oil is its viscosity. For many people, it is obvious that oil with a viscosity of 0W-20 is thin oil, while oil with a viscosity of 10W-60 is a thick oil. These terms may be used colloquially, but it is worth remembering that in practice, things can be quite different.

Everything depends on the conditions in which the oil operates and what properties it has. It may happen that low viscosity oil will become thick at high temperatures, and high viscosity oil will be thin while the engine is running. This is due to the oil's viscosity index.

For example, 15W-40 oil compared to 0W-40 oil will have a higher viscosity at room temperature. At 100 degrees Celsius, both oils will have the same viscosity, and at higher temperatures, which means the hottest parts of the engine, 0W-40 oil will have a higher viscosity than 15W-40 oil.

Experts from Total remind us that seeking oil with the highest possible viscosity is very risky because the manufacturer's engine standards should be the priority. Too low or too high an oil viscosity affects the engine workload and the operation of hydraulic elements such as valve lifters, phase variators, or tensioners.

As a result, an ill-considered oil change can at best lead to an increase in engine noise and a decrease in performance, and at worst - failures, such as stretching of the timing chain.

It is very important that even when choosing oil viscosity independently, only use the range that is approved by the engine manufacturer. Such information can always be found in the user manual.

Andrzej Husiatyński, an expert at TotalEnergies, summarizes: "Modern engine oils are so advanced that they require careful reading of labels by the buyer. Knowing the viscosity of the oil is no longer enough, as even within one, several variants of the oil appear that differ, for example, in manufacturer approvals. Absolutely do not be guided by the appearance of the oil in the packaging, especially as we usually see it at room temperature. Color, transparency, and smell during operation tell us nothing about the lubricating properties and can be misleading".