In most hydraulic components, small orifices and clearances of various geometric shapes govern their operation and influence the components and the hydraulic system's efficiency and dynamic behavior. In applications requiring precise actuator positioning with small and rapid movements, mathematical modeling of fluid flow in control valves (such as servo valves and load-sensing regulators) becomes particularly important. A critical part of this modeling is accurately describing fluid flow through small orifices, especially radial clearances. The most important physical properties of fluids that affect flow through small orifices are density and dynamic viscosity. This paper presents the theoretical basis for determining the change in density and dynamic viscosity of mineral hydraulic oils as a function of the thermodynamic state (pressure and temperature), then gives an analytical expression for determining the flow rate through so-called long radial clearances taking into account the thermodynamic state of the working fluid, and finally describes the installation for experimental measurement of flow rate through small orifices (in the specific case of radial clearances).