Engine oil: The great guide

Engine oil protects the engine from wear. The most important information from oil change to mixing and viscosity to SAE classes, from ACEA specification to oil dilution to the right measurement.


How An Engine Works

Please include attribution to https://engineoilguide.com/ with this graphic.

Why do cars need motor oil?

Engine oil prevents a car’s engine from being damaged while driving. It lubricates the moving parts in the engine – for example the pistons and thus reduces mechanical friction. It also cools the parts that get hot while driving and which the coolant cannot reach, protects against corrosion and cleans the engine of ash, soot and metallic abrasion.

Can I mix motor oils?

Oils of the same specification or quality can be mixed together, but it is not ideal. In exceptional cases – for example, when the oil warning lamp comes on – you can refill inferior oil – according to the specifications of the vehicle manufacturer.

Which engine oil is the right one?

Different engines need different engine oils. This applies to gasoline and diesel engines, but also to different engine variants. The oils are precisely tailored to the individual requirements of the respective manufacturer.

That is why there are many different specifications and corresponding approvals: For example BMW Longlife-04, VW 507.00 or Mercedes-Benz 228.51. Few vehicle manufacturers prescribe general oil qualities according to ACEA (European Vehicle Manufacturers Association) or API (American Oil Institute).

Information on the respective prescribed engine oil classification can be found in the  operating instructions  or in the service book – and can also be found on the  engine oil canister . One should adhere to these guidelines. If not,  the manufacturer’s guarantee is jeopardized .

Why do you have to change engine oil?

Oil is refilled
Refilling oil is easy, but not draining ∙ © Shutterstock / Ensuper

Engine oils wear out and therefore have to be  changed regularly  . The reasons for wear and tear are natural oil aging, the degradation of additives and the viscosity index improver, as well as  contamination  by combustion residues such as soot, sulfur oxides and water, dust from the atmosphere and mechanical abrasion. Filtering or refilling fresh engine oil is not enough. In  petrol engines, which mostly drive short distances, the engine oil can also be  thinned by unburned petrol components. This lowers the viscosity and reduces the lubricity of the engine oil. Oil dilution can also occur in diesel engines with a particle filter.

Oil change: can you do it yourself?

From oil changes on their own  experts advise . In modern cars, the oil drain plugs can often only be opened with special tools, and proper disposal of the oil is also time-consuming. In addition, the drain plug is hardly accessible without a lift or ramp – and a trip to a self-help workshop takes a lot of time. 

Mineral oil or synthetic oil – which is better? 

Mineral oils can be obtained from petroleum by distillation and refining and consist of many different hydrocarbon molecules. During the manufacturing process, the unusable components of the crude oil are sorted out. This process is relatively simple and comparatively inexpensive.

Synthetic engine oil is made from petroleum or natural gas. Here, however, the components are not sorted out, but rather  assembled in a  chemical process so that the desired product is created. In this way, the manufacturers can give the oil special properties – for example, particularly good lubricity at extremely low or high temperatures.

What is longlife oil?

Different motor oils for the car
Known oils: Castrol, Liqui-Moly, Shell, Jet and Meguin ∙ © ADAC eV

Longlife oil consists of high-quality base oils and special additives. They are usually classified according to SAE 0 W or SAE 5 W and are rather  thin . So that they can lubricate the engine parts well despite their thin liquid, they are optimized with special additives. Longlife oil is intended for vehicles with long-life service; So for cars with a long oil change interval specified by the manufacturer. In principle, only the oil quality specified by the vehicle manufacturer may be used with these cars – even when refilling. Since longlife oils are not available from every dealer, you should always have one liter of the corresponding oil with you to refill.

Can you save money with low-viscosity oil?

Low-friction oils, which also include long-life oils, are intended to reduce energy loss due to mechanical friction in the engine. In fact, depending on the outside temperature, operating conditions and engine type, motorists can save fuel with their help: at best, four to six percent on a short trip. Consumption can be up to four percent lower on highways, as well as on city and rural trips.

However, low-viscosity oil is usually more expensive than most standard products. Therefore, the financial saving potential should be a few euros a year. In addition, the oil should only be used if the automaker prescribes or permits it for the respective vehicle.

What happens if you put too much oil in the engine?

If there is too much engine oil in the oil pan, bubbles can form in it. In the worst case, the diesel engine will  pull the oil: The oil will be sucked into the engine and burned, the engine will run out. The result is an engine failure. If you have refilled too much oil, there are two options: If the max display on the dipstick is only just exceeded, the trip to the next workshop should not be a problem. The situation is different if significantly more oil is poured into the tank than prescribed: Then the car stops better. An ADAC breakdown service can help.

When and why must the oil filter be changed?

The engine oil runs through the oil filter  to clean it of  metal debris ,  dust  and  combustion residues before it reaches the lubrication points. To do this, the engine oil is sucked out of the oil pan by the oil pump with the engine running and pressed through the filter with pressure. From there it is pumped to the lubrication points of the engine.

After prolonged use  , the filter  becomes clogged with residues . If the oil filter is not changed in time, the oil can no longer flow through the filter and be cleaned in the process. The engine is lubricated anyway: because when the filter is clogged, a bypass valve opens and lets through as much oil as necessary.

However, it then flows uncleaned and together with all harmful particles into the engine. When changing the filter, you should therefore stick to the intervals specified by the vehicle manufacturer.

Why do petrol and diesel cars with a particulate filter need special engine oil?

Particulate filters are supposed to filter dust and soot out of the car exhaust – for example the notorious fine dust. Engine oil residues also get into the filter from the car engine. The particle filter must be burned free at regular intervals – for example after driving 1000 km – so that it does not become blocked.

Unfortunately, some engine oil residues cannot be burned as well as soot. Drivers of cars with a particle filter should therefore use an engine oil that burns as ash-free as possible. These oils are referred to as low SAPS or low ash engine oils.

The requirements for these engine oils are laid down in the ACEA specifications C1 to C5. The C stands for passenger car diesel engines with particle filters. Some car manufacturers have set their own requirements for their cars with particle filters. At BMW this engine oil is called Longlife-04, at Mercedes-Benz MB 229.31 or MB 229.51 and at Volkswagen VW 507.00.

Oil level – how to measure correctly

Oil level is checked
Before measuring: Be sure to wipe the dipstick ∙ © ADAC / Gerd George

An incorrect oil level can cause engine damage. Therefore, it should be checked regularly with the engine warm. Wait about two minutes after stopping the engine. During this time, the oil from the engine has run into the oil pan. Only then pull out the dipstick, clean it with a cloth and then insert it again until it stops. Then pull out again and read the status. If the oil is between the “Min” and “Max” marks, everything is OK. Oil must be refilled under “Min”, best drained through a workshop via “Max”. One reason for high oil levels can be oil thinning.

Rising oil level due to oil thinning 

When the oil level rises, it is usually due to oil dilution. This phenomenon occurs in diesel cars, which are mainly driven on short distances. The cause: Dust and soot collect in the particle filter. It must be burned down again and again so that the filter does not become blocked.

Because the necessary temperature of up to 700 degrees Celsius cannot be reached on short journeys, additional fuel is injected into the cylinder, i.e. the combustion chamber. However, it does not always burn completely and then mixes with the engine oil. The result: the oil level rises. Diluted oil has less lubrication. The wear in the engine increases and with it the risk of damage.

Therefore, you should check the oil level – especially for diesel cars in short-haul mode – regularly and change the engine oil if necessary.

What does engine oil viscosity mean?

Viscosity is the viscosity of liquids and gases. Water, for example, has a low viscosity: it is thin and therefore very fluid. At a temperature of 20 degrees it has a viscosity value of 1.

The colder a liquid is, the more viscous it becomes, the warmer the less viscous. This is particularly important with engine oils: after all, engine oil has to lubricate all parts in the engine very quickly, even when starting cold in winter. However, if it becomes too viscous in the cold, it cannot be pumped into the engine quickly enough. This would result in engine damage.

On the other hand, engine oil must not become too thin at high temperatures – i.e. when the engine is running at full speed. Because then it loses its lubricity. This leads to higher friction in the engine, an even higher temperature and ultimately also engine damage.

The viscosity of engine oil is specified with the SAE viscosity classes, e.g. SAE 5W-20, SAE 20W-50 or SAE 15W-40. The instruction manual tells you which SAE your vehicle needs.

What are the SAE viscosity grades?

SAE is short for the Society of Automotive Engineers. This organization defines various technical standards for the automotive industry. These include the SAE viscosity grades. This value is given on motor oil canisters with two numbers: e.g. SAE 10W-40.

The number before the W describes the flow properties of the engine oil when it is cold. The smaller the number, the better the engine oil can flow in the cold, and the faster it reaches the important lubrication points in the engine. The number after the W indicates the flow properties of the engine oil when it is warm. The higher this value, the better the lubricity of the engine oil is retained when the engine is hot.

Today’s multigrade oils contain viscosity improvers. They ensure that the oil maintains its lubricity at both high and low temperatures.

Basically, when buying motor oil, always choose the oil that meets the specifications of the manufacturer of your car.

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