The main task is to lubricate the motor to reduce mechanical friction on the moving parts. Otherwise metal would seize on metal in a very short time. Motor 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 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 the cooling of heat-stressed parts that the coolant cannot reach (e.g. pistons). However, the engine oil is also used for fine sealing between piston, piston rings and cylinder running surfaces. Further tasks are the protection against corrosion, the cleaning of the engine (combustion residues and metallic abrasion are absorbed and kept in suspension) - as well as the power transmission (e.g. in hydraulic tappets).
The particulate filter can only eliminate soot, but not the combustion residues from the engine that enter the filter. Sooner or later these will clog it. There are two possible solutions: Either the filter is dimensioned large enough so that it can absorb the oil ash over a certain mileage, or an engine oil is used that burns as ash-free as possible. The sulphate ash is used as a measured value for this. An oil formulation with reduced ash content is characterised by special additives that contain less sulphur and phosphorus. These oils are known as low-SAPS or low-ash engine oils. The requirements for these engine oils are laid down in the ACEA specifications C1 to C4 (C = car diesel engines with particle filter; see ADAC bulletin 'Fachinformation Motoröl'). 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 nature and performance of modern engine oils are based on different base oils or blends. In addition, additives are used which 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. 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 longest known and used base oils. They consist of hydrocarbon compounds of various shapes, structures and sizes. Mineral oils are monograde oils by nature 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 straight-line molecules, which are not found in crude oil. Synthetic oils already have a multi-grade characteristic, which is why viscosity index improvers can be used much more economically. Due to their uniform structure, they respond better to the effectiveness of additives, so that special performance characteristics can be achieved more easily. Fully synthetic oils have some significant advantages over mineral oils:
• better cold start at low temperatures • less wear due to faster supply of the lubrication points • Lubricating film does not break off even under higher loads and high temperatures.
Engine oils that reduce mechanical friction losses compared to normal multigrade oils are called low-friction oils. These are synthetic and hydrocrack oils with a high viscosity index and low evaporation loss. Low-friction oils are mostly in the SAE ranges 0W-30, 0W-40, 5W-30 and 5W-40. Their fuel-saving potential depends on the following factors:
• Oil properties (viscosity, friction-reducing additives) • Driving conditions (short-haul, long-haul, partial, full-load operation) • Engine type and construction (petrol, diesel engine) • climatic conditions (engine temperature).
The following savings are possible according to studies by the ADAC: • Short distance traffic: 4 to 6 percent • Mixed city/overland: 2 to 4 percent • Motorway: up to 2 percent
Whether the use of a fuel-saving motor oil is worthwhile compared to a conventional product must be examined in each individual case.
Longlife oils also belong to the category of low-friction lubricants, mostly in the SAE range 0W-30 or SAE 0W-40, and are prescribed in vehicles with Longlife service. A control unit uses the data from various sensors (for oil level, brake wear, speed, consumption, rpm) to calculate the engine load and the next inspection date accordingly. This means that 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 when the vehicle is subjected to greater stress, e.g. in city traffic and short distances. A display in the instrument cluster indicates the next oil change and maintenance service, depending on the time or distance travelled. With the Longlife concept, only the Longlife oil quality approved by the vehicle manufacturer for the respective engine may be used - even when refilling. The use of other, non-approved oils leads to increased wear and can, under certain circumstances, lead to engine damage. If no Longlife oil is used, the control unit must be reprogrammed to fixed inspection intervals. Longlife oils are not available everywhere. It is therefore recommended to carry one litre of the required quality for refilling in the car.
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, metallurgy and the design of the engine components. When selecting the engine oil, one should therefore always follow the manufacturer's specifications and only then orientate oneself towards the price. For the required oil quality, the vehicle manufacturers determine the classification and specification to be used. Besides the SAE viscosity class, the API classification and the ACEA specification are of primary importance here (see ADAC bulletin '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, one should therefore make sure when selecting the oil that the information in the operating instructions or the service check booklet corresponds to that on the oil container. Oil guides' from the mineral oil manufacturers, can provide assistance in finding 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 range (scooters, lawnmowers) are taboo for the common four-stroke petrol and diesel engines. Engine oils for the same engine concept, on the other hand, can generally be mixed with each other, also synthetic with mineral. However, the performance characteristics prescribed by the car manufacturers must always be observed. If an engine is filled with an oil of lower performance than prescribed, this can lead to damage. On the other hand, topping up oil of a higher performance is no problem.
Despite significant leaps in development, engine oils are still subject to a certain amount of wear and tear and must therefore be changed after a certain period of time. The reasons for this are natural oil ageing, degradation of additives and contamination by combustion residues (e.g. soot, sulphur oxides, water), dust from the atmosphere and mechanical abrasion. Refilling with fresh engine oil alone is not sufficient. In petrol engines with frequent cold starts, the engine oil can also be diluted by unburned petrol components. This lowers the viscosity and reduces the lubricity of the engine oil. Oil thickening occurs mainly in diesel engines through soot absorption of the engine oil, but also in petrol engines under extreme thermal load. The thickening causes cold start difficulties, especially in the cold season, and may 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 construction of the engine and the quality of the prescribed engine oil.
Although synthetic peak oils often have much higher power reserves, the change interval must not be extended compared to the prescribed oil quality, as the design requirements are lacking. These include the design of the oil filter, which must be adapted to the longer dwell 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 bulletin 'Never change oil again - are off-line oil filters useful?
Motor 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 periods, components of the chemical additives dissolved in the oil may precipitate. Of much 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 the current standards. This applies to both synthetic and mineral engine oils. In opened containers, the storage time for engine oil should not exceed half a year. Each container 'breathes' as a result of changing ambient temperatures, i.e. it draws in outside air with the corresponding air humidity. This reduces the performance of the engine oil through chemical-physical reactions with the additives.