Transport and material handling is a sum of technological processes in the wood-processing industry. Hydraulic drives are one of the drive options used as a basic part of the transport – handling machines. Contamination control involves preventing contaminants from entering a hydraulic system and placing filters in strategic locations throughout the system to trap any contaminants to find their way into the fluid. But for critical equipment, a successful contamination control program must also include regular assessment of the hydraulic fluid cleanliness. It must be often done every two to six months or after every 500 or 1,000 hours of operation, depending on the equipment duty cycle, operating environment, and how critical it is to overall operation. Pressure forces in hydraulic drives are transferred to hydraulically-operated devices which also serve to lubricate the elements, displace and remove heat from the system. To increase the service life of hydraulic drives, hydraulic oil must meet individual physical characteristics. To ensure reliability, timely diagnosis of hydraulic oil is required. Some experts also recommend that fluid to be tested immediately after any maintenance event that exposes the hydraulic system to the external environment. This could occur when a hose or other component is replaced or fluid is added to the reservoir. Fluid replenishment can be particularly troublesome because new fluid is notorious for being dirty – often from improper storage and handling practices. The article deals with the methodology of measuring the basic physical properties of hydraulic oil by the respective diagnostic devices directly on the hydraulic drive of the bend saw in the full operation of the machine. The methodology suggested enables making decisions for improvement of the hydraulic oil state.