## 2 Materials and Methods  
The LO used in the experiment was Marinol CB-30 RG1230 [35; 36] viscosity grade SAE 30 LO designed for trunk-piston marine engines and Orlen Efecta Diesel Biodiesel oil [26; 37]. Samples of fresh oil and samples of ULO for testing were taken from the lubrication system of the ZUT Zgoda Sulzer 5 BAH22 engine installed at the Marine Power Plant Laboratory of the Maritime University of Technology in Szczecin. The calendar operation time of the ULO at the time of collection was 2 years, and the engine operating time was 60 h.  
A comparison of the basic physicochemical properties and elemental composition of the LO and DO used in the experiment is shown in Tables 1 and 2, respectively, while the contaminations present in the ULO are summarized in Table 3. All values presented in Tables 1 and 3 were determined in our laboratory using the methods given in the first column of the tables. Content of chemical elements presented in Table 2 was determined with use of ASTM D6595-17 (2022) method [38]. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Parameter (Measurement Method Used in**} & \multirow{2}{*}{**Unit**} & \multicolumn{3}{c}{**Value**} \\ \cline{3-5}  & **This Experiment)** & & **FLO** & **ULO** & **DO** \\ \hline Density at 15 \({}^{\circ}\)C & & & & \\ (PN-EN ISO 12185:2002 [39]) & kg/m\({}^{3}\) & 890.6 & 889.6 & 828.3 \\ \hline Kinematic viscosity at 40 \({}^{\circ}\)C (PN-EN ISO 3104:2021-3 [40]) & mm\({}^{2}\)/s & 110.310 & 102.740 & 2.480 \\ \hline Kinematic viscosity at 100 \({}^{\circ}\)C (PN-EN ISO 3104:2021-3 [40]) & mm\({}^{2}\)/s & 11.770 & 11.420 & 1.067 \\ \hline Viscosity index (ASTM D2270-10(2016) [41], & & & & \\ [PERSON] calculator [42]) & – & 94.13 & 94.13 & 274.35 \\ \hline Initial boiling point (Flashpoint Pensky–Martens apparatus [43]) & \({}^{\circ}\)C & 276.0 & 240.5 & 175 \\ \hline Flashpoint in closed cup (PN-EN ISO 2719:2016 [44]) & \({}^{\circ}\)C & 216.0 & 190.0 & 56.0 \\ \hline Derivated Cetane Number (ASTM & & & & \\ D7668(2017) [45]) & – & N/A & N/A & 52 \\ \hline Lubricity—HFFR wear scar diameter (PN-EN & & & & \\ ISO 12156-1:2018 [27]) & \(\mu\)m & 212 & 149 & 331 \\ \hline HFFR friction coefficient (HFFR V.1.0.3 procedure [34]) & – & 0.123 & 0.121 & 0.191 \\ \hline Relative oil film resistance drop (HFFR V.1.0.3 procedure [34]) & \% & 100 & 100 & 76 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Properties of tested lubricating oils (fresh and used) and diesel oil. \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Chemical Element**} & \multicolumn{3}{c}{**Content (ppm)**} \\ \cline{2-4}  & **FLO** & **ULO** & **DO** \\ \hline Fe & 0.0 & 3.2 & 0.0 \\ Cr & 1.0 & 2.6 & 0.0 \\ Pb & 7.4 & 14.7 & 3.7 \\ Cu & 0.0 & 0.0 & 0.0 \\ Sn & 7.1 & 8.0 & 18.0 \\ Al & 5.8 & 6.0 & 5.3 \\ Ni & 7.5 & 15.2 & 4.6 \\ Ag & 0.7 & 0.6 & 0.3 \\ Si & 4.1 & 11.5 & 2.1 \\ B & 0.8 & 2.1 & 0.1 \\ Na & 4.9 & 7.4 & 4.6 \\ Mg & 0.1 & 0.8 & 0.2 \\ Ca & 4.8 & 7.3 & 3.1 \\ Ba & 0.0 & 0.0 & 0.0 \\ P & 0.0 & 0.0 & 0.0 \\ Zn & 1.1 & 4.1 & 0.0 \\ Mo & 2.1 & 2.3 & 1.0 \\ Ti & 0.0 & 0.0 & 0.0 \\ V & 0.0 & 0.0 & 0.0 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Chemical composition of tested Marinol RG 1230, tested lubricating oils (fresh and used) and Orlen Efecta Biodiesel oil (measurements of 9 May 2024 with use of ASTM D6595-17 (2022) method [38]). The experiment allowed for the testing of mixtures with concentrations of DO in LO. Samples of fresh and ULO were diluted with DO at concentrations of 0, 1, 2, 5, 10, 15, and 20% m/m. An RADWAG WPs 510/C/2 (Radom, Poland) precision laboratory balance with a minimum graduation \(d\) of 0.001 g served to prepare the samples. The calculated type-B standard uncertainties \(u_{B}\)[50] for the mass fractions of DO in the tested LO were \(C=1\%\) m/m--\(u_{B}(C)=0.026\%\) by weight, \(C=2\%\) m/m--\(u_{B}(C)=0.010\%\) m/m, \(C=5\%\) m/m--\(u_{B}(C)=0.002\%\) m/m, and \(C\geq 10\%\) m/m--\(u_{B}(C)=0.001\%\).