## 2 Materials and Methods
Lubricity is the behavior of a lubricant during boundary friction; therefore, it is an ensemble characteristic since lubricating properties do not depend only on the parameters of the oil, but also on the mating elements (i.e., structural materials, contact geometry, and the type of movement) and their load [22]. The measurements were made with a high-frequency reciprocating rig (HFFR) tribometer, model HFFR V1.0.3 [34] (PCS Instruments, London, UK), to comply with ASTM D6079-22 [51] and PN-EN ISO 12156-1:2018 [27]. The size of the wear scar was determined using a standard optical (metallurgical) upright microscope HFR2 [52] (PCS Instruments, London, UK). The schematic diagram and laboratory setup of the HFFR device are shown in Figures 2 and 3. During the test, the ball oscillates at a constant speed and makes a consistent stroke while the contact point of the ball with the plate is fully immersed in the liquid being tested. The test conditions, such as the metallurgical properties of the plate and ball, fluid temperature, load, oscillation frequency, stroke length, and external conditions, are specified in the standard [27]. The average wear scar diameter (\(WSD\)) formed on the surface of the test ball provides a measure of the lubricity properties (lubricity of the test fluid). Additionally, the device used provides information on the average value of the friction coefficient \(\mu\) and the relative oil film resistance drop \(r\), which measures the reduction in oil film thickness during the test execution. \begin{table}
\begin{tabular}{c c c} \hline
**Parameter (Measurement Method Used in This Experiment)** & **Unit** & **Value** \\ \hline Content of DO (ASTM D8004-15 (2023) [46]) & \% m/m & 0.01 \\ Content of water (PN-EN ISO 12937:2005+API:2021-11P [47]) & \% m/m & 0.05 \\ Determination of total sediment (PN-ISO 10307-1:2001 [48]) & mg/g & 0.35 \\ Determination of the content of the contaminant particulates (PN-ISO 4405:1994 [49]) & mg/100 mL & 77.1 \\ Content of chemical elements (ASTM D6595-17 (2022) [38]) & ppm & See Table 4 \\ \hline \end{tabular}
\end{table} 
Table 3: Content and contaminants of tested ULO (based on own measurements). Figure 2: Schematic of the HFFR apparatus [31]: 1—test ball, 2—test plate, 3—oscillating rod, 4—test fluid reservoir, 5—test mass, 6—heating bath, 7—support, 8—controllable base, 9—elastic lock, 10—main frame, 11—elastic connector housing, 12—electromagnetic vibrator, 13—counterweight, and 14—force transducer. The average wear scar diameter \(WSD\) in micrometers (\(\mu\)m) is determined as the average value of the wear scar \(x\) (\(\mu\)m) measured perpendicular to the direction of ball oscillation and the wear scar \(y\) (\(\mu\)m) measured parallel to the direction of ball oscillation [54]. In order to be able to compare the results of the lubricity measurement in different external conditions, the standard measure of lubricity is corrected to a standardized water vapor pressure of 1.4 kPa. The wear scar diameter \(WS_{1,4}\) indicator is calculated using the following formula [55]:  
\[WS_{1,4}=\frac{x+y}{2}+60\cdot(1.4-AVP), \tag{1}\]  
where \(AVP\) (kPa) is the mean absolute vapor pressure of the tested lubricant liquid during the test [55]. \[AVP=\frac{\left(\frac{h_{1}\cdot 10^{8.01735-\frac{1270.844}{251.864+11}}}{750} \right)+\left(\frac{h_{2}\cdot 10^{8.01735-\frac{1270.584}{251.864+12}}}{750} \right)}{2}, \tag{2}\]  
where \(h_{1}\) (%) signifies the air humidity in the laboratory room at the beginning of the test, \(h_{2}\) (%) is the air humidity in the laboratory room at the end of the test, \(t_{1}\) (\({}^{\circ}\)C) is the temperature in the laboratory room at the beginning of the test, and \(t_{2}\) (\({}^{\circ}\)C) is the temperature in the laboratory room at the end of the test. For the measurement method used, the measurement error was not determined since lubricity is not a fundamental physical quantity and was not a property of the liquid being tested [51].