Danger! The impact of using the wrong oil in your car's engine

Combustion engines, whether petrol or diesel, have changed a great deal in recent years. Development has aimed at achieving the best performance with the highest possible efficiency. Emissions regulations are becoming ever more restrictive, and every measure goes.

Thus we have seen the spread of “downsizing“, or engine capacity reduction, with the arrival of small 3-cylinder engines with displacements of around 1 litre. Forced induction has already become standard. And it has been necessary to introduce new exhaust-treatment systems: catalytic converters, particulate filters…

Many engines have even had their sumps reduced to a minimum, so they run on the smallest possible amount of oil. Maintenance intervals have also been extended, with oil change periods lengthened in many cases. This is possible, to a large extent, because of the evolution that lubricants have also undergone. Oil is now designed not only to keep the engine and its components from failing, but is also required to help reduce emissions.

Thus low-viscosity lubricants, synthetic oils, fully synthetic oils and lubricants with highly controlled specific additives have emerged.

The role of oil in the engine

Understanding all the functions oil performs in an engine will help us better grasp what happens if the wrong lubricant is used. The main one is reducing friction and wear between mechanical parts by creating a layer of lubricant between them. But it has many more:

• Lubricates, reducing friction between metal parts
• Provides a seal between the piston and the cylinder
• Reduces the engine’s operating temperature
• Has detergent properties, cleaning the engine
• Neutralises the acids produced during combustion
• Minimises corrosion
• Protects the anti-pollution devices
• Reduces the risk of auto-ignition or connecting-rod knock in modern direct-injection petrol engines

That’s no small thing. Imagine the damage an unsuitable oil can do to an engine. You can go into more detail here to learn why oil is so important for your car’s engine. In other articles we have also covered how to change the oil yourself, and we have told you which are the best synthetic 5W30 oils, which tend to be the most widely used.

The important thing is always to follow the specifications the manufacturer gives for a particular engine, and never go below them if you want to avoid problems with the engine… or with any of these components.

EGR: synthetic and low-evaporation oils

A real headache for many turbodiesel owners, the EGR valve is responsible for reducing nitrogen oxide emissions. It works by feeding part of the exhaust gases, which contain less oxygen, back into the intake, lowering combustion temperature.

But the EGR is a real “magnet” for the carbon deposits and residues carried by the exhaust gases, so it is very common for it to get dirty and end up malfunctioning. To keep the EGR from failing, the best option is to use quality, 100% synthetic oils with a low evaporation rate. A 100% synthetic oil is better than a semi-synthetic one, and it will reduce the formation of residues that would shorten the life of the EGR or cause it to malfunction.

The turbocharger: the highest-quality oil is essential

The turbocharger is one of the most common solutions for boosting engine performance and reducing consumption. But because of its characteristics and operation, correct lubrication is of vital importance here. Furthermore, a turbocharger failure is very expensive to repair.

Turbochargers are becoming smaller and capable of spinning faster, up to 200,000 rpm. And the temperature of the exhaust gases passing through the turbo when the engine is under full load easily reaches 800 degrees Celsius. As a result, the temperature of the oil lubricating the turbocharger shaft can reach 200 degrees. Any oil begins to struggle to maintain its stability at such high temperatures. Bear in mind that the flash point of a good oil is around 230 degrees Celsius.

Imagine what happens if the oil does not flow correctly through the turbo shaft: it evaporates, leaving by-products that stick to the shaft. This is what can end up causing the dreaded turbo “seizure”.

So to avoid problems with the turbocharger, the best choice is a fluid oil with high thermal and oxidation stability, as well as high resistance to carbonisation and shearing.

It often happens that an engine already high in mileage, which normally runs on 5W30 oil, the grade recommended by the manufacturer for that car, starts consuming oil. The workshop recommends switching to 10W40, which is cheaper… and it will reduce oil consumption. And it does, because of its higher viscosity (resistance to flow) both cold and hot. But the result is that a 10W40 oil reduces turbo lubrication by 30% compared with a 5W40 when the engine is cold, and by more than 22% when the engine is hot. So think carefully before saving a few euros on each oil change, especially if your car has a turbo.

Particulate filters and engine oil

Another component of modern engines that causes many problems is the particulate filter, especially in diesel cars. Petrol particulate filters are also becoming more common lately, but because petrol engines burn more cleanly in terms of ash, these cause fewer problems.

We have already covered the problems that can make you come to hate your car’s particulate filter. But using an oil that is not the correct one will only make them worse. Besides the particles that build up in the filters as a result of incomplete combustion, which the filter stores for later removal through a regeneration process, there are other particles that are very harmful to filters: the sulphated ash produced by using unsuitable oils.

This ash forms a kind of “white cement” that clogs the filter, causing constant regeneration cycles that shorten its useful life, damage the pressure sensors, and eventually clog the filter completely, preventing it from self-cleaning.

In this case you should not look only at viscosity. For example, an oil such as Motul 8100 X-Cess 5W40 has a viscosity equivalent to Motul 8100 Clean 5W40, and even the same flash point. But the former is an ACEA A3/B4 with 1.1% sulphated ash, while the latter is an ACEA C3 with 0.8% sulphated ash. If the manufacturer recommends an ACEA C3, C4 or C5 for your car’s engine, do not go below that figure, or you will put the particulate filter at risk.

An oil that meets ACEA A3/B4 is not a bad oil; it simply has more sulphated ash, and it is not suitable for cars with particulate filters.

SCR catalysts, oil and AdBlue

Another exhaust-treatment system in diesel engines is the SCR catalysts with AdBlue injection, which reduce nitrogen oxides. Here you can read a complete guide to AdBlue, its care, tips… As far as the lubricant is concerned, for these catalysts to work properly, oils with a controlled content of phosphorus and sulphur are required.

An oil manufacturer would use larger amounts of these elements in the lubricant formulation were it not for the presence of this device. SCR catalysts are highly sensitive, and they cannot tolerate exhaust gases with a high presence of phosphorus and sulphur. This gives, once again, an idea of why it is so important to stick to the oil specifications each manufacturer recommends for a given vehicle.

Oil for hybrid and plug-in hybrid cars

Hybrid vehicles and plug-in hybrids, which are increasingly common on the market, are also a special case. Even though they have a combustion engine that, in theory, works like the engine of a conventional car.

However, in these cars, where the engine is continuously subjected to start-stop cycles, the oil is subjected to temperature loads that do not occur in cars with a conventional combustion engine without electrification. During phases when the car runs on the electric motor, or when the engine is disconnected from the transmission and the car coasts, the oil cools down very quickly.

In these cases it is especially important to use very low-viscosity oils, and never to switch to an oil with higher viscosity. It is common to find oils for this type of vehicle with 0W16 viscosities, which should not be used in conventional combustion-engine cars that are not hybrids either. Here you have more details on choosing the best oil for your hybrid car.

Related information of interest

• The best synthetic 5W30 oils for your car’s engine
• Car oil: synthetic or semi-synthetic?