Engines with higher mileage and "thicker" oil

It is very common for a driver or mechanic to decide to change the oil „for a thicker one”, selected at their own discretion. Such a move is supposed to, according to old theories and the belief of some mechanics, extend the life of the engine or reduce oil consumption when the car exceeds 100,000 km of mileage. The effect can be quite the opposite of what was intended. TotalEnergies warns against such behaviour and explains the risks.

At the outset, it is worth clarifying that the popular term „thicker” refers to the viscosity grade of the oil. Drivers or mechanics want to change the viscosity class of the oil recommended by the manufacturer, e.g. from 0W-20 to 0W-30 or 5W-40. The most common argument given in such situations is usually: „I will not lubricate the engine with water-like oil” or „factory recommendations are ecological nonsense, everyone knows that the „thicker” the oil, the better”.

There are also other theories that have been heard, e.g. that when the engine exceeds 100,000 km, it should be filled with oil with a viscosity of at least 5W-40 or 10W-40, because „thicker oil protects the turbocharger better”. Unfortunately, in most cases, instead of extending the life of the engine, this shortens it.

„Density” and viscosity – basic knowledge

The colloquial „density” of oil is not a correct term from a technical point of view. Density is a determination of specific weight and has little to do with the viscosity of the oil. 0W-20, 5W-30 are viscosity grades. It is the viscosity that determines how easy the oil flows at a certain temperature. The higher the viscosity, the greater the internal resistance, the poorer the heat dissipation and the longer the time it takes for the oil to reach all engine components at the start-up (both morning and start-stop).

Where does the confusion come from?

In theory, a 0W-30 oil will have a higher viscosity at high temperatures than a 0W-20 oil. Therefore, you may think that after warming up the engine, the oil film will be thicker, more „durable”. In practice, it doesn't have to be this way. Laboratory tests show that in the latest, high-performance engines, oils with higher viscosities... they cool the engine worse, which increases the temperature of some of its components (rings, turbocharger bearings), so changing from 0W-20 to 0W-30 does not do anything. Yes – the temperature gauge may still show about 90 degrees, but it is about local temperature increases in selected, unmonitored places of the engine, where 0W-30 oil will be worse to accept and dissipate heat worse than 0W-20 oil. In modern engines, which are more thermally loaded, this function of the oil is particularly important.

Other serious consequences

It is important to remember that the viscosity of the oil is only one of the parameters that are given by the manufacturer. The most important thing, however, is to select the oil in accordance with the manufacturer's homologation. If the manufacturer allows two variants of oil in the user manual, e.g. 0W-20 and 0W-30 and indicates what approvals/approvals of the oil are to be met in both cases – everything is OK. In this case, these oils can be used interchangeably.

It is worse if the manual does not provide for such a possibility and changing the viscosity class to a higher one will be the „arbitrary” of the user. In such a case, pouring oil into the engine without the appropriate approval/approval from the manufacturer will have very serious consequences.

An unauthorised oil may not meet the standards in terms of:

• protection against pre-ignition
• LSPI phenomenon in turbo petrol engines with direct injection,
• protection against sludge and deposits in the engine (too low soot dispersion),
• cooling of engine components (rings, turbo, etc.)
• compatibility with valve hydraulics, variable valve timing systems, etc.
• compatibility with exhaust gas treatment systems (DPF, GPF).
• compatibility with additives protecting the timing belt working in oil or protection of the timing chain against elongation.
• may have a different composition of anti-wear additives.

And what about the engines used in hybrids?

Here, we are dealing with a specific case. These engines very often run underheated. Not only do they turn on and off every now and then, but they often go into high revs, „cold” right away. In such cases, we need instant lubrication and specially selected additives that will work at lower temperatures. That's why applying the „thicker oil protects better” principle will do exactly the opposite, especially in turbocharged hybrids.

Summary

Arbitrary changes in viscosity or deterioration of engine oil quality, contrary to the vehicle manufacturer's recommendations, are a simple way to shorten the engine life.

On the oil market, there are a lot of different „truths” and theories. One of them, unfortunately, is that older engines with higher mileage should be treated with a higher „density” oil. As we have already mentioned, it is about higher viscosity. Unfortunately, such changes are usually associated with a deterioration in the quality of the oil and failure to meet the manufacturer's homologation. In such cases, the engine in which the combustion efficiency begins to deteriorate (which generates more oil pollution) deteriorates the quality of the oil even more. This is the shortest way to shorten its lifespan. Therefore, as always, we remind you that you should use the oil in accordance with the manufacturer's approval, regardless of whether the engine has 100 or 300 thousand km. If it starts to „consume” oil, you should not „fix” this condition by pouring oil with a higher viscosity, or adding special „sealants”. You should continue to use the proper oil in accordance with the manufacturers approval and even reduce the drain intervals, as the oil in a high-mileage engine will have more severe operating conditions due to its contamination. And changes to the viscosity grade of the oil can only be carried out within the range specified by the manufacturer and in accordance with the approval. Andrzej Husiatyński, Technical Department, TotalEnergies Marketing Polska Sp. z o.o.