Expert contradicts popular beliefs about engine oil color and viscosity: what drivers should know

One unruly concern for drivers is the black coloration of engine oil in a diesel engine, especially when this occurs soon after an oil change. This can cause unrest as it might indicate the oil wasn't replaced. The scenario is common in engines that have covered considerable mileage.

However, TotalEnergies experts suggest that such a situation shouldn't be a cause for worry, but rather a reason to rejoice. The black coloration of the oil indicates its success in absorbing soot and preventing the accumulation of deposits in various parts of the engine. It does not negatively impact the oil's physical properties, given it was appropriately selected for the engine — a function that is expected of the oil.

Another surprising occurrence to drivers is highly clear oil, often so transparent it's scarcely noticeable. This is usually the case with new cars just out of the dealership. Tilt it correctly to catch the light and discern its level. But how does this apply in the context of older cars?

Clear oil is most often observed in engines powered by autogas. LPG fosters cleaner combustion and minimizes pollution, maintaining the oil's freshness. It is indeed favorable for drivers, yet, they should avoid using the oil's color as a guide to schedule oil changes.

Even in oil that appears clean but has been in use, invisible chemical impurities accumulate over time. These pollutants adversely impact the oil, deteriorating its lubricating properties. Here, the main concern is sulfur, which is present in LPG.

Sulfur incites oil acidification and a decline in its lubricating attributes, which is why certain manufacturers offer oil products composed to prolong its lifespan. For vehicles powered by LPG, Total experts recommend more frequent oil changes if standard oil is used, than in those running on gasoline.

Drivers also concern themselves with oil viscosity as an index for its density. Many would instinctively perceive 0W-20 oil as thin and 10W-60 oil as thick. Even though these terms can be colloquially accurate, the reality could be different.

The oil's behavior depends on the working conditions and its inherent properties. For instance, under high temperatures, low-viscosity oil could thicken, while high-viscosity oil might thin during engine operation. This is attributed to the oil's viscosity index.

A 15W-40 oil, compared to a 0W-40 oil, will demonstrate higher viscosity at room temperature. Yet at 100 degrees Celsius (212 degrees Fahrenheit), both oils will share similar viscosity. Furthermore, in hotter parts of the engine, 0W-40 oil exceeds 15W-40 oil's viscosity.

Total experts caution that seeking oil with the highest viscosity can be risky, emphasizing it's best to stick to the engine manufacturer's standards. Variations in oil viscosity can negatively affect engine operational resistance and the performance of hydraulic components such as valve lifters, variable valve timing systems, and tensioners.

Consequently, ill-considered oil changes could lead to increased engine noise, reduced performance, and in more severe scenarios — failures like an overextended timing chain.

It's therefore crucial to adhere to the manufacturer's acceptable oil viscosity range — this is outlined in the user manual.

"Modern engine oils have advanced to a level that warrants careful label reading by the buyer," advises Andrzej Husiatyński, an expert from TotalEnergies. "It's no longer sufficient to know the oil's viscosity alone, as different variants even within a single type may differ notably — manufacturer's approvals being a case in point. We certainly should not solely rely on the appearance of the oil in its packaging, particularly since this is typically assessed at room temperature. The color, clarity, and smell of oil during operation provide no insight into its lubricating properties and could be misleading."