## 1 Introduction
There is no consensus among researchers on how fuel dilution affects the lubricity of engine oil. According to some research [8] carried out in specific engine operating conditions where the start-stop cycle protocol was used; fuel-to-oil content reached 6%; however, the lubricating oil presented only a minimal effect on friction during this time and an inconsistent effect on wear when compared to fresh oil. However, in contrast, other studies have found a reduction in the oil load-carrying capacity by increasing the fuel content, starting from the hypothesis that the effectiveness of high-pressure additives in the engine oil has been reduced [4]. Another study conducted on SAE 5W-50 engine oil, where 6% gasoline was added to it and tested under various load conditions, showed that friction and wear increased by 3-16% and 2-10%, respectively [9]. Other studies, in which up to 15% of diesel oil was added to the lubricating oil of a five-cylinder car engine with a capacity of 2700 cm\({}^{3}\)[10], showed a significant impact on the viscosity of the engine oil (its significant reduction) but no visible increase in engine wear was observed. This research aimed to specify the impact of fuel oil on two key components of engine oil: the base oil and the antiwear additive. It is well established, for example, that fuel dilution in engine oil can lead to a multitude of detrimental effects on engine performance [11]. These include the occurrence of boundary lubrication, accelerated reduction of lubricating oil pressure, excessive wear of engine components like piston rings, cylinder liners, bearings, and crank-pins, a reduction in engine performance, and as a result of all the above, a reduction in engine lifetime. Additionally, in the modern maritime industry sector, there are standards for marine fuels that must be met. These standards include the level of sulfur content in fuel, which in the case of many ports and sea areas cannot exceed 0.1%. To achieve such low sulfur content in marine fuel, it has to be removed during the refining process, which has been carried out for many years. But, to achieve such a low sulfur content, components that provide natural lubrication are also to be removed. However, low sulfur content in fuel and consequently in exhaust gases is also beneficial for the atmosphere and the marine natural environment. Nevertheless, diesel engine parts such as the fuel pump and fuel injectors require proper lubrication as they are susceptible to excessive and premature wear. A way to improve the lubricating properties of ultralow sulfur fuels is various additives that improve lubricity and restore lost wear protection. Due to their structure, the particles of these additives create a lubricating film on the surfaces of the working parts. As a result, friction between these surfaces is reduced and the service life of these parts is extended. The use of such additives has become an economical and necessary solution when using fuels with an ultralow sulfur content. However, these additives improve the lubricating properties only; they do not completely eliminate the deterioration of the fuel lubricatingproperties. In consequence, the diesel engine parts such as the injection pump and the injectors are still deprived of additional lubrication and are exposed to excessive wear. This study's aim was to analyze changes in the operational properties of lubricating oil used in the crankcase of the fuel injection pump of a marine engine. This paper focuses on oil lubricity, mainly on what changes are determined by conditions where there is no contact of lubricating oil with exhaust gases or unburned fuel during operation. Oil can be contaminated only by \"fresh\" fuel entering the oil through leaks in the precision pair (barrel-plunger) in this pump. The possibility of contamination by water is also very limited due to the lack of fuel injection pump cooling and small crankcase volume capacities where condensation could occur.