Oil viscosity
Viscosity of motor oils
Viscosity characterizes the fluidity of the oil. Depending on the temperature, the oil tends to decrease in fluidity. As the temperature increases, the oil becomes more fluid. This worsens the oil's ability to adhere and withstand high contact pressures. When cooling, the oil thickens, fluidity decreases, and internal friction increases. Because of this property, it is necessary to use motor oil with a viscosity that changes with temperature within the smallest possible limits.
When starting a cold engine, the oil must be sufficiently liquid to avoid overloading the engine and to allow the oil to quickly penetrate all areas where lubrication is needed after starting.
The fluidity of oil is equivalent to its internal friction and is designated by SAE units (Society of Automotive Engineers - Association of Automotive Engineers), for example SAE 30, SAE 10. A high SAE value means that the oil is thick, a low value characterizes liquid oil. However, oil viscosity does not fully characterize the lubricating properties of the oil.
All-season oils
All-season oils are preferable for use in car engines. The advantage of all-season oils is that they do not need to be selected depending on the season. They are manufactured on the basis of liquid single-season oil (type 10W). When heated, this oil is stabilized by a so-called thickener, which ensures the appropriate lubricating properties of the oil at all temperatures. When using all-season oil, modern grades with a wide temperature range of viscosity should be used, for example 10W-40, 15W-50.
The letter W (winter) in the SAE designation characterizes the suitability of the oil for use in winter.
Viscosity grades and oil usage range
Since the range of use of adjacent SAE classes overlaps, short-term temperature fluctuations can be ignored. Mixing oils of different viscosity classes is allowed if oil top-up is required and the ambient temperature no longer corresponds to the viscosity class of the oil poured into the engine.
For long-term outside temperatures below –20°C, it is recommended to use SAE 5W-20 oil.
Warning: CD motor oils, which the manufacturer designates as oils for diesel engines, are not suitable for gasoline engines. There are oils that can be used in two types of engines, in which case both designations are indicated on the oil container, for example SG/CD.
Warning: Additional oil additives, regardless of their type and purpose, should not be added to the oil.
Motor oils
For modern engines, only D oils should be used. HD oils are oils with increased lubricating properties, improved by various additives. These additives provide better anti-corrosion protection, more favorable conditions against oxidation, the least tendency to form slags, the best change in viscosity, cleaning and dissolving properties. Cleaning and dissolving additives not only reduce the formation of residues in the engine, but also have the ability to dissolve these residues and all other contaminants in the engine oil and keep them in a suspended state so that when changing the oil, these contaminants are removed together with the drained oil.
The quality of HD oils is determined by the API system (American Petroleum Institut). European manufacturers also adhere to this system.
The oil grade is designated by two letters. The first letter characterizes the area of application: S – Service, intended for carburetor engines; C – Commercial, designed for diesel engines.
The second letter identifies the grade of oil in alphabetical order.
The highest grade oils according to API classification are SG for carburetor engines and CD for diesel engines.
Petrol engines
- A - all-season oils with improved antifriction properties, specification VW 500 00.
- B - all-season oils, specification VW 501 01;
- all-season oils, API-SF or SG specifications.
Diesel engines
- A - all-season oils with improved antifriction properties, specification VW 500 00 (for turbocharged diesel engines only in mixture with specification VW 505 00).
- B - all-season oils, specification VW 505 00 (unlimited for all diesel engines);
- all-season oils, API-CD specification (for turbocharged diesel engines only if absolutely necessary for topping up);
- all-season oils, specification VW 501 01 (for turbocharged diesel engines only in mixture with specification VW 505 00).
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