Abstract Sufficient oil supply of all machine elements in gearboxes is usually required to avoid damage and to reduce friction during operation. However, dip lubrication and injection lubrication always result in hydraulic gear power losses. Especially in high-speed and dip-lubricated gearboxes with high oil levels, churning losses can represent a significant portion of the total power loss. Current analytical and empirical approaches for examining the churning losses are often limited to certain constraints and operating conditions. Furthermore, they do not provide any information about the oil distribution. CFD (Computational Fluid Dynamics) methods offer a very flexible way to investigate the oil distribution and the churning power losses, with almost no restrictions on the housing shape and operating conditions. In this paper, a CFD model based on the Finite Volume Method (FVM) is built to investigate the oil distribution and the churning losses inside a single-stage gearbox of the FZG back-to-back efficiency gear test rig. Thereby, a three-dimensional finite volume simulation model considering two-phase flow is applied. The results are compared with measurements of the churning losses and with high-speed camera recordings. The comparisons show very good agreement and high potential for predicting the oil distribution and the churning losses in modern transmission systems.