Test verification of the condition of hydraulic oil of construction machines following the guidelines of ISO 4406 and NAS 1638

The article aims to present the testing of hydraulic oil testing in accordance with the requirements of ISO and NAS standards and the results of oil testing from two selected construction machines, i.e. Liebher 566 first class wheel loaders. The article also takes into account the influence of hydraulic oil microfiltration on the cleanliness class and relative humidity, which is an indirect measure of the amount of water in the oil. Oil microfiltration was carried out regularly and allowed for the improvement of the oil condition, which is understood as a decrease in the cleanliness class by removing solid contaminants and water from the oil through the use of membrane filters. The hydraulic oil tests carried out by the authors also take into account the assessment of the oil condition determined before and after microfiltration.