## 4 Discussion  
The obtained test results concerning the kinematic viscosity of berry seed oils were compared with the technical data of other oils on the basis of the available literature. The data concerning the hydraulic HL 32 oil were read from the PN-ISO 11158:2012 standards [31]. The guidelines described in this standard specified the range of kinematic viscosity for oils used in hydraulic systems at a temperature of 40 \({}^{\circ}\)C within the range of 28.8-35.2 mm\({}^{2}\)/s. At 40 \({}^{\circ}\)C, the kinematic viscosity of the HL 32 oil is 32.6 mm\({}^{2}\)/s. For the tested plant oils, the value of the kinematic viscosity was higher than 32.6 mm\({}^{2}\)/s by 3.9 mm\({}^{2}\)/s and 6.7 mm\({}^{2}\)/s for the strawberry seed oil and chokeberry seed oil, respectively. At a temperature of 100 \({}^{\circ}\)C, all of the tested oils showed a significantly higher kinematic viscosity, ranging from 25.4% to 55.9% in comparison with the viscosity of the HL 32 oil, which is 5.4 mm\({}^{2}\)/s. The kinematic viscosities of the plant oils shown in Table 1 at 40 \({}^{\circ}\)C were similar to those of other plant materials. The similarities in kinematic viscosity ranges were also noticeable at the temperature of 100 \({}^{\circ}\)C. An exception from this was the castor oil, whose kinematic viscosity was significantly higher at both 40 \({}^{\circ}\)C and 100 \({}^{\circ}\)C.  
The highest values of the viscosity index (VI) were obtained from the blackcurrant seed oil (235) and raspberry seed oil (237). A lower value was obtained from the chokeberry seed oil, and the lowest came from the strawberry seed oil. The comparison of the obtained results to the HL 32 oil data showed that the tested plant oils had higher viscosity indexes than the HL 32 oil by 54.5% to 230.1%. A viscosity index higher than 200 shows the possibility of the correct performance of such an oil within a wider temperature range. The anti-wear berry seed oil tests conducted with a four-ball apparatus showed that the wear scar diameter ranged from 0.865 mm to 0.885 mm, as shown in Figure 3. These values can be considered average because, for this class of industrial oils, the wear scar diameter does not exceed 0.5 mm, while for other industrial oils (including rapeseed and soy oil), the diameter is approx. 0.7 mm [32; 33; 34]. Figure 9: The steady-state characteristics of the hydrodynamic clutch (type KG VTXZ74T) for the tested working fluids. — = blackcurrant seed oil and - - - = hydraulic oil. Figure 4 shows that the resistance to oxidation of the examined oils ranged from 25 min to 68 min. Chokeberry seed oil had the lowest thermo-oxidative resistance (25 min), while raspberry seed oil had the highest thermo-oxidative resistance (68 min). These values are really low in comparison with other plant oils, such as rapeseed oil (110 min). The examples in Table 3 concerning the corrosive effect show that corrosion did not occur for three out of the four samples. In the case of chokeberry seed oil, matting and discoloration of the plate occurred, matching the 3a pattern shown in Figure 5. This is a negative result, according to PN-ISO 11158:2012 standards [31]. The maximal corrosion degree allowed by this standard is two, under the milder temperature conditions of the test. Figure 6 shows that the pour points of the tested plant oils ranged from \(-25\,^{\circ}\)C to \(-22\,^{\circ}\)C. The pour point temperature range, determined on the basis of the tests, was similar to the pour point temperature ranges of most hydraulic oils. The characteristics of the hydrodynamic clutch M = f(ik), obtained for different rotational speeds of the hydraulic clutch's input shaft, were nearly the same for both the HL 32 oil and the blackcurrant seed oil, as shown in Figure 9. It should be noted that the blackcurrant oil was used with no correction to its composition in terms of suitability for work in hydraulic components.