What you need to know about lubricants and workshop chemicals!
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The main task is to lubricate the motor in order to reduce mechanical friction on the moving parts. Otherwise, metal would seize up on metal in a very short time. Engine oils must ensure lubrication under all operating conditions occurring in the engine:
In cold conditions, they should be as thin as possible (so that the starter can start the engine more easily and the oil reaches all lubrication points as quickly as possible). At the same time, the lubricating film must not break off even under full load and must be able to withstand high temperatures. Another important task of the engine oil is to cool heat-stressed parts that the coolant cannot reach (e.g. pistons). The engine oil is also used for fine sealing between pistons, piston rings and cylinder running surfaces. Other tasks include protection against corrosion, cleaning the engine (combustion residues and metallic abrasion are absorbed and kept in suspension) and power transmission (e.g. in hydraulic tappets).
The particulate filter can only remove soot, but not the combustion residues from the engine that get into the filter. Sooner or later, these clog it up. There are two possible solutions: Either the filter is dimensioned so large that it can absorb the oil ash over a certain mileage, or an engine oil is used that burns as ash-free as possible. Sulphated ash is used as the measured value for this. An oil formulation with a reduced ash content is characterized by special additives that contain less sulphur and phosphorus. These oils are referred to as low-SAPS or low ash engine oils. The requirements for these engine oils are defined in the ACEA specifications C1 to C4 (C = passenger car diesel engines with particulate filters; see ADAC technical information on engine oils). Some manufacturers have extended their specifications in this respect, e.g. BMW Longlife-04, MB 229.31 and MB 229.51, VW 507.00.
The properties and performance of modern engine oils are based on various base oils or blends. In addition, additives are used to improve the properties of the lubricants through chemical and/or physical action. Only a balanced formulation of base oil and additive components results in a high-performance engine oil. As a rule of thumb, the higher the proportion of additives in the engine oil, the higher the quality - and also the price. Mineral oils are the base oils that have been known and used for the longest time. They consist of hydrocarbon compounds of different shapes, structures and sizes. Mineral oils are naturally monograde oils and can be produced relatively easily and cheaply from crude oil by distillation and refining. Like mineral oils, synthetic oils are also produced from crude oil, only the production process (synthesis) is more complex and expensive. While mineral oils consist of many different hydrocarbon molecules, fully synthetic oils are largely made up of uniform, linear molecules that do not occur in crude oil. Synthetic oils already have a multigrade characteristic, which is why viscosity index improvers can be used much more economically. Due to their uniform structure, they respond better to the effect of additives, so that special performance characteristics can be achieved more easily. Fully synthetic oils have some significant advantages over mineral oil.
Engine oils that reduce mechanical friction losses compared to normal multigrade oils are referred to as low-friction oils. These are synthetic and hydrocrack oils with a high viscosity index and low evaporation loss. Low-viscosity oils are usually available in the SAE ranges 0W-30, 0W-40, 5W-30 and 5W-40. Their fuel-saving potential depends on the following factors:
Longlife oils also belong to the category of low-friction lubricants, usually in the SAE 0W-30 or SAE 0W-40 range, and are prescribed in vehicles with a longlife service. A control unit uses the data from various sensors (for oil level, brake wear, speed, consumption, engine speed) to calculate the engine load and thus the next inspection date. VW, for example, allows maintenance intervals of up to 30,000 km for petrol engines and up to 50,000 km for diesel engines. These maximum mileages are reduced if the vehicle is subjected to greater stress, e.g. in city traffic and on short journeys. A display in the instrument cluster indicates the next oil change and maintenance service depending on the time or distance driven. With the Longlife concept, only the Longlife oil quality approved by the vehicle manufacturer for the respective engine may be used - even when topping up. The use of other, non-approved oils leads to increased wear and can possibly result in engine damage. If longlife oil is not used, the control unit must be reprogrammed to fixed inspection intervals. Longlife oils are not available everywhere. It is therefore recommended that you keep a liter of the desired quality in the vehicle for topping up.
Different engine concepts place fundamentally different demands on the engine oil. The correct oil quality for the respective engine and its change intervals are therefore determined by the vehicle manufacturer in numerous test runs, taking into account the oil filter system, the metallurgy and the design of the engine components. When selecting engine oil, you should therefore always be guided by the manufacturer's specifications and only then by the price. The vehicle manufacturers determine the classification and specification to be used for the required oil quality. In addition to the SAE viscosity class, the API classification and the ACEA specification are particularly important here (see ADAC leaflet 'Fachinformation Motoröl'). However, there are also vehicle manufacturers who define their own test standards, compliance with which is then documented on the packaging by the oil suppliers (e.g. VW 507.00). In order not to lose any warranty or goodwill claims, you should therefore make sure when selecting the oil that the information in the operating instructions or in the service checkbook matches that on the oil container. Oil guides' from the mineral oil manufacturers can help in the search for the 'right' oil.
Oils for different engine concepts (e.g. petrol or diesel) should never be mixed due to the different requirement profiles. Oils from the two-stroke sector (scooters, lawnmowers) are taboo for standard four-stroke gasoline and diesel engines. Engine oils for the same engine concept, on the other hand, can generally be mixed with each other, even synthetic with mineral. However, the performance characteristics specified by the vehicle manufacturer must always be observed. If an engine is filled with an oil with a lower performance than prescribed, this can lead to damage. On the other hand, topping up with oil with higher performance is not a problem.
Despite considerable leaps in development, engine oils are still subject to a certain amount of wear and therefore need to be changed after a certain period of time. The reasons for this are natural oil ageing, the degradation of additives and contamination from combustion residues (e.g. soot, sulphur oxides, water), dust from the atmosphere and mechanical abrasion. Topping up with fresh engine oil alone is not sufficient. In gasoline engines with frequent cold starts, the engine oil can also be diluted by unburned gasoline components. This reduces the viscosity and the lubricity of the engine oil. Oil thickening occurs primarily in diesel engines due to soot absorption of the engine oil, but also in gasoline engines under extreme thermal stress. The thickening causes cold starting difficulties, especially in the cold season, and can make an additional oil change necessary. The oil change intervals recommended by the vehicle manufacturers must therefore be observed. They are determined depending on the design of the engine and the quality of the prescribed engine oil.
Although top synthetic oils often have significantly higher performance reserves, the change interval must not be extended compared to the prescribed oil quality, as the design requirements are not met. This includes the design of the oil filter, which must be adapted to the longer residence time, as well as the metallurgy and design of the engine components. The oil change intervals specified by the manufacturer should therefore be observed. Information on long-term oil filters can be found in the ADAC leaflet 'Never change the oil again - are bypass oil filters a good idea?
Engine oils can be stored for a relatively long time in properly sealed original containers. The manufacturers recommend not exceeding a period of three to a maximum of five years. If stored for longer, components of the chemical additives dissolved in the oil can settle. Of far greater importance, however, is the fact that technical progress in engine development inevitably requires engine oils with ever higher performance. Older, stored engine oils cannot meet these performance requirements. Engine damage can occur because the oil no longer meets current standards. This applies to both synthetic and mineral motor oils. The storage time for engine oil in opened containers should not exceed six months. Every container 'breathes' due to changing ambient temperatures, i.e. it draws in outside air with the corresponding humidity. This reduces the performance of the engine oil through chemical-physical reactions with the additives.