In multi-task machine tools, hydraulic oil makes both lubricating and power transmission functions, realizing a lubricating film that, due to its constant thickness, permits to limit vibrations and allowances, contributing to the machine precision. Usually very fine pressure filters are installed to protect the system. A customer of us, called us claiming that filter cartridges clog very frequently, thinking that this was due to a poor filter cartridge quality. We tested the fluid both before and after the filter cartridges: going outside from the filter cartridges, the fluid was really clean but coming to the filter cartridges, it was highly polluted with solid particles.