## 1 Introduction  
Lubricity is a characteristic of a tribological system that describes the ability of lubricant molecules to adhere to lubricated surfaces and form permanent layers on them that reduce friction. When two different oils share the same viscosity, the one with superior lubricity yields less friction loss under analogous operating conditions, thereby minimizing wear between mating parts [1]. High lubricity is crucial for engine reliability, especially during boundary or mixed lubrication scenarios--such as engine maneuvering, starting, and stopping. In particular, marine main propulsion engines with a fixed propeller are exposed to such scenarios [2]. Lubricity refers to how effectively a lubricant reduces friction and wear. Unlike a material property, it cannot be directly measured. Instead, specific tests are conducted to evaluate a lubricant's performance within a particular system. Various types of tribometers are used to determine lubricity. In this experiment, we used a high-frequency reciprocating rig (HFFR) tribometer, and the corrected average wear scar diameter \(WS_{1.4}\) test conditions were taken as a measure of wear. Both the operation of the device and the method of determining the size of the wear trace are described later in the article, and the correct wear scar diameter \(WS_{1.4}\) in test conditions was taken as a measure of lubricity. The lubricity of oils for heavy-duty internal combustion engines is achieved by using extreme pressure (EP) additives in the form of organic compounds of phosphorus, sulfur, oxygen, and other elements. Oil lubricity is also improved by natural oil aging products such as asphalt, resins, or organic acids, added in low concentrations. These contaminants contribute to the improved adhesion of oil to lubricated components of lubricating oils [1]. On the other hand, when the oil contains organic impurities above 2.1% m/m or asphaltenes above 0.15% m/m, the wear intensity of the tribological pair does not decrease [3], as shown in Figure 1. With an increase in contamination in the lubricating oil (LO), new risks of filter contamination, clogging of oil pipes, deposition of lakes and carbon deposits on lubricated surfaces, deterioration of heat transfer, galling of piston rings, and overheating of engine components occur. All these elements increase the wear and tear of engine components and shorten their life [4]. Engine life can be influenced by an important contaminant, which is fuel that mixes with oil due to a malfunctioning injection system, drainage system, fuel leakage, and damaged or excessively worn piston rings, pistons, and cylinder liners. Experience shows that dilution of lubricating oil (LO) with diesel oil (DO) is unfavorable to the engine, causing issues including deterioration of engine performance, shortening of oil life, and reduction in engine reliability and safety [5; 6]. An excessive, progressive dilution of oil with fuel can lead to significant wear and tear, and, ultimately, to serious engine failure. The analysis of such phenomena can be supported by control computational models (computational tribology) that enable the automation of the oil condition checking process using computer systems [7]. Figure 1: Relationship between the engine component wear and contaminant particulate concentration in lubricating oil (compiled based on ref. [3]). The oil film is crucial for reducing the coefficient of friction at tribological junctions. The main problem with the dilution of LO with DO is the reduced viscosity of the LO since diesel oil (DO) has a much lower viscosity than LO. Therefore, the dilution of LO can cause a deterioration of the lubricating properties of the given oil and a reduction in the strength of the oil film, which, in turn, contributes to the intensification of the wear of cylinder liners and bearings [8; 9], as well as the possibility of causing premature ignition in the engine cylinder [10].