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  1. From the point of view of proper functioning of an engine as well as its appropriate service life, motor oil is an important and integral element of an engine and it has to match it as far as the technical quality standards are concerned.

    The most important tasks of oil include:
    • reducing the friction between the moving surfaces,
    • protection against corrosion,
    • heat transfer (heat dissipation from the hot elements of an engine),
    • sealing the ‘piston-piston rings-sleeves of the cylinder of the engine’ system,
    • preventing the formation of deposits, transporting them to a filter,
    • cooperating with the engine sealants in order to prevent leaks to the environment
  2. Taking the above mentioned tasks into consideration, a designer of an engine indicates what kind of motor oil should be used. He usually provides both the viscosity and the quality class as well as the oil change interval. Next to a general rule of choosing oil, which is used in accordance with the producer’s requirements, one needs to take the following facts into account:
    • in the case of a car that has a considerable mileage, one should not change its oil for the one that has a higher class than it was previously used,
    • synthetic and half-synthetic oils are designed for cars that are new or have a low mileage. For similar reasons as have been mentioned before, one should not use them in older, worn out engines,
    • oil mixing should only take place in the case of oils of the same class, mixing two different quality classes lowers the higher oil class,
    • oil change interval should take place according to the car producer’s recommendations and is about every 15 thousand km, and recently even every 30 thousand km for petrol engines and every 50 thousand km for diesel engines with turbocharger. This was brought about by the development of engines and a huge progress in oil technology. Almost all producers recommend a more frequent oil change in more difficult exploitation conditions.
  3. Exploitation efficiency of motor oils was normalized in the international scale through disseminating the classification :
    • Viscosity classification according to SAE – the current SAE viscosity class differentiates 6 classes of winter oils: 0W, 5W, 10W, 15W, 20W, 25W (W – winter). The lower the number before the W letter, the lower the temperatures that the oil can be used in. Additionally, 5 classes of summer oils were differentiated: 20, 30, 40, 50, 60. Most oils that are currently produced are the all-season ones, marked with e.g. 10W/40, which can be used both in winter and in summer. In winter, in low temperatures, they make the warm-up easier, and in summer, they ensure better lubrication of friction pairs. One can assume that for the all-season exploitation, the lower the W marking the better the oil, whereas the second marking has to meet the requirements of a producer.
    • Quality classification according API – divided into 2 groups: S (service) – engines with a positive ignition. They are marked with a two-letter code (SA, SB, SC, SE, SF, SG, SH, SI, SL, SM).
    • C (commercial) – engines with a compression ignition. They are marked with a two-letter code (CA, CB, CC, CD, CD-II, CE, CF, CF-II, CG-4, CH-4, CI-4), the further the second letter in the alphabet, the higher the oil quality.
    • Quality classification according to ACEA (European Automobile Manufacturers Association), introduced a new classification for oils and it was determined that:
      • Petrol engine oils will be marked with the letter - A (A1-98, A2-96, AC-98),
      • Oils for the lightly burdened diesel engines – B (B1-98, B2-98, B3-98, B4-98),
      • Fully-burdened diesel engines – E (E1-96, E2-96, E3-96, E4-98).
What is also important is the fact that each engine consumes oil. As it is partly combusted, certain consumption is a natural phenomena. In newer engines, its consumption may vary from 0.05 to 0.3 litres per 1000 kilometres. In older engines, the consumption increases as the piston rings wear out and allow more oil to pass or it gets to the combustion chamber through the worn out valve sealants. For this reason, you have to comply with the requirements of a producer as far as the interval of oil change is concerned.