The article presents the issue of microfiltration of hydraulic oil from metallic impurities and water that deteriorate the properties of the oil. Tests carried out on oils have shown that periodic oil replacement in machines or vehicles in accordance with their maintenance documentation ensures that the system is emptied of 95% of impurities from the old oil before pouring the new one. The remaining part, i.e. 5% of impurities, together with the oil remaining on the walls of the pipes or inside other hydraulic devices, will mix with the new oil, reducing its cleanliness class. Periodic replacement of hydraulic oil will not allow for complete cleaning of the system. In extreme cases, poor oil condition may cause disruptions in the operation of the machine with the hydraulic system or its downtime for oil replacement or repair. Filtering the oil in the system while the machine is running will clean the oil from impurities and water, extend the oil's service life, without disabling the hydraulic system. The article includes, among others: The justification for the use of microfiltration of hydraulic oil with portable filtering devices in order to extend the service life of the oil until the next replacement is presented. The main purpose of the article is to present the methodology for testing hydraulic oil in accordance with the requirements of ISO, NAS and SAE in order to assign the oil to a given cleanliness class. Additionally, the results of testing the oil cleanliness class in various periods of operation of the machine with the hydraulic installation on a portable measuring device were presented.