## 1 Introduction  
Energy conservation and environmental responsibility are becoming increasingly important across industries. Marine transportation, a significant contributor to global fuel consumption, is actively seeking solutions to reduce fuel usage and emissions. One approach focuses on improving engine performance to minimize fuel consumption and carbon footprints. Studies have shown that wear and friction between engine components are the main culprits behind energy loss [1]. One of the key strategies to enhance the tribological behavior of marine engines involves using high-quality fuels and lubricants. Proper lubrication is essential to performance, durability, and reliability, especially for marine engines where it is not possible to easily call services or deliver necessary spare parts once a vessel or rig platform is at sea. Lubricating oil is a composite substance of hydrocarbon molecules containing numerous atoms of carbon, hydrogen, oxygen, and others. Due to this composition and atomic structure as well as special additives, the oil film is resistant to flow due to its adhesion to wetted surfaces. The viscosity of lubricating oil is an important indicator of its condition due to its rapid changes in this parameter in the event of lubricant failure [2, 3]. If the viscosity is low, less energy is needed to shear the oil and cool it but, on the other hand, low viscosity causes the oil film to be thinner and therefore more easily and quickly ruptured, resulting in dry/mixed friction. A good understanding of the viscosity of engine oil is significant because it is one of the most important properties of lubricating oil and also diesel engine conditions. Moreover, another similarly important lubricating oil condition indicator is the content of any impurities that dissolve in oil or mix with it as a result of being flushed out by the oil during its flow through various parts of the engine. The contaminants that leak into oil during operation constitute a critical factor that reduces the lubricating properties of engine oils. Many different contaminants can be present in oils that cause changes in oil viscosity, possible chemical changes in additives, and changes in oil lubricity. The two most common contaminants in engine oils are fuel and water [4, 5]. In turbocharged and direct injection (DI) engines, unburned fuel can be attached to the cylinder walls. This fuel then leaks through the piston rings and contaminates the oil in the crankcase. Research has shown that this problem may be particularly severe in DI engines and the fuel content in the oil may reach up to 9%. As a result, the oil viscosity may drop significantly, even by 30% [6]. The concentration of fuel in oil at the TDC (top dead center) position of the piston ring on the wall of the cylinder liner is much higher than in the oil sum. Using gas chromatography and mass spectrometry, as much as 21.9% of fuel was found in the engine oil in the top dead center (TDC) zone after one hour of engine operation [7]. For example, in truck vehicle diesel engines, incomplete filter regenerating cycles can result in significant amounts of unburnt fuel being mixed with the lubricating oil. This contamination significantly degrades the oil's physical-chemical and lubricating properties. In marine diesel engines, the fuel dilution in the lubricating oil is mostly caused by malfunctioning fuel injectors. These injectors can malfunction in several ways, such as leaking fuel or failing to properly atomize it. Both scenarios lead to excessive amounts of fuel entering the oil. The dilution of fuel oil in lubricating oil in the trunk piston engines is a normal process, provided that the quantity of fuel does not exceed a few percent. But it is difficult or ambiguous to determine this value due to the different operating conditions of engines (i.e., different loads), especially in marine engines. Moreover, different oils and therefore different properties are used in these engines.