## 3 Results and Discussion
### Viscosity and Viscosity Index  
Viscosity and Viscosity Index similarly, to the measurement of the operating oil density and the estimation of the quantity of fuel leaking through the fuel pump into the oil lubricating it were presented in work [5]. To ensure data accuracy, each oil sample underwent multiple tests (at least five) for both: density and kinematic viscosity following the ASTM D445 [24]. The average value from these tests, as shown in Figure 4, was used for further analysis. It was noted that the oil from individual samples had the lowest permissible kinematic viscosity values in the range from 3.8 to 21, 9 cSt at 100 \({}^{\circ}\)C [25]. However, the viscosity values determined for samples with an operating time of 8 h and more are lower than the limit values specified by SAE (Society of Automotive Engineers) [26]. The results show a significant drop in the oil's kinematic viscosity after just one hour of operation (Figure 5). This is probably caused by a substantial dilution of the oil with fuel. As the engine continues to run, the rate of fuel contamination and the corresponding decrease in viscosity slow down. Notably, the viscosity difference between the oil samples collected after 24 and 36 h of operation is relatively minor [5]. Some other oil samples also reached similar values of viscosity, which can be observed on the graph by overlapping the lines i.e.  
The minimum lubricating oil viscosity requirements vary among marine engine manufacturers. Some specify absolute kinematic viscosity values (cSt) at a specific temperature, while others define limits based on the percentage decrease in viscosity. The minimum absolute viscosity for certain manufacturers is 5.6 cSt at 100 \({}^{\circ}\)C [5]. Considering this value as a general benchmark (optimistically), the tested oil falls below the limit after just one hour of operation. The average viscosity measured for the oil sample collected after the first hour is 5.6 cSt and it continues to decrease with further operation. However, it is important to note that this value remains above the SAE standard limit of 3.8 cSt as defined by the ASTM D445 standard.  
Figure 4: Approximate change in fuel oil quantity in fuel injection pump Lubricating Oil.  
The viscosity index (VI) is a dimensionless indicator that determines how the viscosity of a fluid changes with the temperature changes. It is mainly used for lubricating oils and greases. A higher viscosity index means a liquid is more stable and will keep a more consistent viscosity at both higher and lower temperatures. This is better for oil used in machinery. Viscosity index grades are generally taken as [27]:  
* Low--under 3,
* Medium--35 to 80,
* High--80 to 110,
* Very high--above 110.  
The standard ASTM D2270 [28] practice for calculating viscosity index from kinematic viscosity at 40 and 100 \({}^{\circ}\)C was used to determine the viscosity index of the tested samples. The calculation was made by using an open viscosity index calculator [29]. As shown in Figure 6, where the VI decreases considerably to about 25% of the initial value. However, the oil from almost all samples maintained a very high or high VI despite the decline. Only the last samples with a working time of 32 h went down to the VI medium level.  
Figure 5: Measured change in viscosity at different temperatures for different oil samples.  
Figure 6: Viscosity index change in fuel injection pump lubricating oil based on VI calculation.