The research problem concerning oils used for lubricating piston combustion engines is still very current and important. The proper selection of oil and its properties have a significant impact on engine reliability and durability, their efficiency, effective operating parameters, fuel consumption, environmental impact, and the proper operation of the turbocharger and exhaust system. The work concerned determining the effect of temperature and operating time on the dynamic viscosity of oils: mineral, semi-synthetic, and synthetic, used in compression-ignition engines (diesel engines). Dynamic viscosity tests were conducted for new oils, after a mileage of seven thousand kilometers, and after a mileage of fifteen thousand kilometers. The range of temperature measurement conditions used was from 0 to 50 °C and the shear transmission rate was 1000 s⁻¹. This range allows the oil to be preserved at low and medium temperatures, which are crucial for engine operation during start-up and short operating cycles. As the conducted studies showed, both temperature and operating time have a very large influence on the dynamic viscosity of oils. It was demonstrated that as the operating time of the oils in the engine increased, their dynamic viscosity decreased, and increasing the viscosity measurement temperature results in smaller absolute changes in it.

The genesis of the conducted research was the intention to determine how the rheological properties of engine oil change upon the addition of a coolant. The aim of the study was to assess the effect of coolant addition to engine oil on its dynamic viscosity. The experiments were carried out for six different fresh engine oils and six of the same oils sampled during routine oil replacement after a vehicle mileage of 10,000 km. Two oils were selected from each of the three primary categories: synthetic, semi-synthetic, and mineral oils. Then, mixtures of engine oils with the addition of coolant were prepared at 0, 5, 10, 20, 30, 40 and 50% (v/v), respectively. The study involved measuring the dynamic viscosity of the samples at 1000 s⁻¹, covering temperatures from 0 to 50 °C. The tests were conducted using a measurement setup equipped with a RHEOLABQC rotational rheometer manufactured by Anton Paar GmbH (Ostfildern, Germany). To investigate the temperature dependence of dynamic viscosity, a GRANT thermostatic bath was coupled with the rheometer. The studies demonstrated that dynamic viscosity strongly depends on temperature, as well as on the type and condition of the engine oil. At 0 °C, the dynamic viscosity of fresh oils ranged from approximately 500 to 700 mPa·s, whereas at 50 °C it decreased to approximately 100 mPa·s. The addition of 5% (v/v) coolant to engine oil resulted in only a slight change in dynamic viscosity. In contrast, a substantial decrease in dynamic viscosity was observed when 50% (v/v) coolant was added to the tested engine oils. The results indicate that coolant, which may enter the engine oil in the event of an engine failure, can significantly deteriorate the rheological properties of this lubricant.