## 4 Discussion  
It seems to be the case that no filtration theory manages to link the filtration efficiency with the media or cartridge operational conditions. However, experimental results commonly illustrate that changes in any of the characteristics of the particles (size, size distribution, shape factor distribution, electrical surface potential, hardness, concentration) and of the liquid (viscosity, pH, density, temperature, viscosity, electrical conductivity, ionic strength, surface tension, face velocity) directly impact the measured efficiency of any kind of media and, ultimately, the filters [14]. According to the authors [18; 19; 20; 21], the basic precondition for the correct function and effective care of hydraulic fluids is a suitably chosen methodology for testing fluids with monitoring of the level of contamination of the working fluid. With an experimental set of pressure measurements with numerical simulation for engines in laboratory conditions, we reduce the time required to perform operational measurements [22]. By verification measurements and results, we demonstrated the suitability of the laboratory test equipment for testing oil filters and monitoring the effect of engine oil contamination on the filtering ability of the oil filter, while changing the measured pressure and flow before and after oil filters. As stated [23], by analyzing lubricating oils during engine operation, it is possible to determine the probability of failure of lubrication of functional parts, determine the suitability of a given type of oil and the oil change interval, together with evaluation of technical condition of functional parts. From all these mentioned aspects, the tribotechnical diagnostics of oils is important not only for monitoring the degradation of the oil itself, determining its current state or the intrusion of undesirable external substances. Diagnosis of the oil or oil level indicates oil contamination and contaminants, warns of possible faults, and allows faults to be prevented [18]. The construction of the laboratory test equipment was carried out based on knowledge from the design of test hydraulic equipment, which they dealt with in the work [15; 16; 24; 25], which created a set of measuring laboratory equipment for testing hydrostatic elements and hydraulic fluids. During the test equipment, it was necessary to solve the question of the location of the oil filters in the hydraulic circuit A and B, together with the question of the service life of the oil filters. All these aspects must be harmonized to ensure the required level of engine oil contamination in the future, when testing the filterability of oil filters. The laboratory test equipment makes it possible to evaluate the level of contaminants and the filtration efficiency of oil filters by monitoring the pressure and pressure drop, monitoring the flow and drop in flow through the inlet and outlet of the test device. The primary reasons for needing pressure drop and particle retention information are that for a new filter medium, the pressure drop as a function of flow rate is used to give the medium permeability. Suppliers measure this using air as the fluid, and the data is helpful in their categorisation. The size of the smallest particle retained indicates it should have a lesser tendency to bleed upon first usage. Moreover, when a filter is in use, the pressure drop is a function of the flow rate through the filter and the amount of particles trapped in or on the surface. Critical for cartridge filters, for cloth-type media, it is important when assessing the likelihood to blind and the rate of cake growth [26]. Standard ISO 16889 \"Hydraulic fluid power-Filters-Multi-pass method for evaluating filtration performance of a filter element\" describes a multi-pass filtration performance test with continuous contaminant injection for hydraulic fluid power filter elements: a procedure for determining the contaminant capacity, particulate removal, and differential pressure characteristics.