The modern automotive market of operating materials for internal combustion engines contains a wide range of varieties of motor oils, gear oils, brake fluids, greases, antifreezes, etc. The main task of the consumer is to be correctly oriented in the choice of such materials, which will favorably affect the technical condition of the engine.

One of the most important indicators in the choice is viscosity, which characterizes the suitability of the oil for use. A certain viscosity is necessary for the formation of a lubricating layer between the friction of surfaces. For this purpose, it is better to use oils with higher viscosity. However, an increase in oil viscosity leads to an increase in unproductive power loss due to friction, efficiency. Hence, the viscosity of the oil should be minimum but sufficient to create fluid friction. In addition, viscosity determines the low-temperature properties of the oil, that is, the ability to provide easy engine starting at low ambient volumes and reliable oil delivery to the main and connecting rod bearings during the starting and warm-up Period of engine operation. With a change in temperature depending on its viscosity.

Motor oils are differentiated into summer, winter and all-season oils by operating temperatures. Distillate components of different viscosity, residual components, mixtures of residual and distillate components, as well as synthetic products (polyalphaolefins, alkylbenzenes, esters) are used as a base oil base. Most multigrade oils are obtained by thickening the low-viscosity oil base with macro-polymer additives [1].

 Depending on the intended use, motor oils are divided into oils for diesel engines, oils for gasoline engines and universal motor oils, which are designed to lubricate both types of engines. All modern motor oils consist of base oils and additive packages that improve their properties.

Engine operation is accompanied by friction of contacting and moving relative to each other surfaces of engine parts. The power developed by the engine is partially used to determine the friction forces.

 The process of friction is accompanied by wear of the rubbing mating surfaces. The work of friction forces turns into heat, which must be removed from the surface of the rubbing parts with a lubricant. The temperature increase in this process negatively affects the efficiency of operation and engine life [2].

The main task of oil is to prevent dry friction of moving internal engine parts and to ensure low friction force with maximum tightness of engine cylinders. It is impossible to produce a substance that would have the necessary properties for this, and at the same time would have stable characteristics in a wide range of temperatures, and the range of operating temperatures of oil in the engine is quite wide.

 The temperature that most motorists observe on the dashboard, and which is commonly called the temperature of the engine, in fact, is the temperature of the coolant, which is really stable in a warmed engine. The oil temperature changes significantly and can reach 130-170  depending on the speed and intensity of movement.

 For each individual engine the manufacturer determines compromise optimal parameters of motor oil. It is these parameters, as the engine manufacturer believes, should provide maximum efficiency (efficiency) with minimum wear of the engine’s rubbing parts under given operating conditions.

Motor oil consists of a base (base oil) and additives that are designed to improve its quality and properties. According to the nature of raw material binding, bases can be either petroleum (mineral) or synthetic [3]. The chemical composition of mineral substrates depends on the quality of the oil, the boiling point of the oil fractions, as well as on the methods and degree of their purification. At direct distillation of fuel oil, oil fractions with low viscosity are extracted from it, such mineral bases are called distillate bases. The base to increase viscosity is obtained from what remains after distillation, tar and semi-tar, these oils are called residual oils.

Improving the quality of mineral oil inputs has limits. Modern engines require oil with properties that exist to provide petroleum refining. Therefore, increasingly common oils are synthetically derived: diether, polyalkylene glycol, fluorocarbon, silicone and others. They perform the same functions as mineral oils, but do so at a higher level of quality. Synthetic oils with exceptionally good viscosity-temperature characteristics. These are, firstly, much lower than mineral, pour point (-50  to -60 ) and a very high viscosity index, that is, a relatively small change in viscosity, depending on changes in temperature, which makes it very much easier to cold Start the engine. Secondly, they have a higher viscosity with operating functions above 100, which makes the oil film separate friction surfaces and does not degrade under extreme thermal conditions [4].

Other advantages of synthetic oils include high thermo-oxidative stability, that is, low tendency to form deposits and varnishes (varnishes called clear, very strong, virtually insoluble films composed of oxidation products applied to hot surfaces) and low vaporization and combustion compared to mineral Oils. It is also important that synthetics require the introduction of a minimum amount of thickening additives, and especially its high quality grades do not need such additives at all, of which these oils are very stable as the additives are destroyed.

All these properties of synthetic oils contribute to the reduction of wear of parts. In addition, their resource exceeds the mineral resource by 5 or more times.

The main factor limiting the use of synthetic oils is their high cost. They are 3-5 times more expensive than mineral oils. In this regard, many companies produce semi-synthetic oils, the concentration of synthetics in which is 25-50%. This compromise is very successful: in terms of quality and price, semi-synthetic material is located between synthetic and mineral oils. Where exactly, depends on the amount of synthetic component.

The pour point is the ultimate temperature at which an oil loses its mobility. Oils with a pour point of -15  or higher in the summer. If the pour point is -20  or below, oils are winter. The pour point characterizes to some extent the limit temperature at which a cooled engine can be started. However, the ambient temperature is independent of the pour point of the oil.

Antiwear properties characterize the ability to reduce load by reducing the input power to overcome friction. These properties are based on viscosity and viscosity-temperature characteristics, lubricity and oil purity. Detergent dispersing properties are divided into detergent properties and dispersing properties. Detergent properties characterize the ability of the oil to provide the necessary cleanliness of parts and resist varnishing on hot surfaces, as well as prevent adhesion of carbon compounds [5]. Dispersing properties characterize the ability to prevent adhesion of carbonaceous particles, to hold them in a state of stable suspension and to destroy large particles of oxidation products when they appear.

 Antioxidant properties determine the stability of the oil on which the life of engine oils depends, characterizing their ability to retain their original properties and withstand external effects during normal operations. The oxidation resistance of engine oils increases with the introduction of antioxidant additives.

Anticorrosive properties depend primarily on the content of sulfur compounds, organic and inorganic acids and other oxidation products in them.  Corrosive wear of parts is also determined by the initial value of alkalinity and its rate of change. The more the oil works, the lower the alkalinity. Therefore, the alkalinity value is included among the indicators of oil quality.

Ash content in oil allows to judge the amount of non-combustible impurities in oils without additives, and in oils with additives about the amount of additives introduced. Ash content in oils, not additives, is not more than 0,02 -0,025 wt.%. In oils with additives the ash content should be not less than 0.4%, and high quality grades of oils should be not less than 1.15-1.65% by weight [6].

Content of mechanical impurities and water. There should be no mechanical impurities in oils without additives, and in oils with additives their value should not exceed 0,015 wt.%. And mechanical impurities should not have an abrasive effect on friction surfaces. Water in engine oils should be absent. Even a small amount of water causes the destruction of additives, there is a process of formation of sludge.

Additives are used to give motor oils new properties or changes. Additives are:

1. antioxidants — increase the antioxidant stability of oils;
2. anti-corrosive — protect metal surfaces from the corrosive effects of acid and sulfur-containing products;
3. dispersants — add deposits of oxidation products to metal surfaces;
4. anti-wear, extreme pressure, and antifriction properties that improve the lubricating properties of oils;
5. depressors — reduce the pour point of oils;
6. profiles — prevent oils from foaming.

Dynamic viscosity is the drag force of two lubricant layers of 1 cm² in an area spaced 1 cm apart and moving at a speed of 1 cm/s relative to each other. Kinematic viscosity is defined as the ratio of dynamic viscosity to fluid density.

Viscosity properties are the most important parameter by which an oil is selected. The viscosity value determines the lubricating and anti-wear properties of the adsorbed oil boundary film, energy input for cold engine starting and circulation. This parameter depends on where it is located and depending on how it occurs [7]. Friction surfaces. Among the conditions: starting at low scale, including: the critical viscosity of the oil viscosity at which the engine power generated by the engine, the necessary power required to overcome the frictional resistance due to the oil viscosity.

In the article were considered issues of purpose and principle of action of engine oil in piston internal combustion engines, as well as the impact of its main characteristics on the technical and economic performance of the engine and its starting properties. So the classification of engine oils is considered, the item about viscosity of engine oil as the main parameter influencing the hydrodynamic mode of friction of cylinder-piston group of the engine and its crankshaft bearings is singled out separately.