This study focuses on the phenomenon of oil trapping in the tooth spaces of spur gears. It presents the first obtained results in order to provide an estimation of the power losses due to the compression of the oil trapped between the inter teeth meshing spaces in a high-speed transmission for internal and external spur gears. It first allows, to model the inter teeth spaces at any time and offers limits for the trapped fluid volume and axial and radial discharge surfaces. The discretization impact result is discussed.Then, the influence of certain design and operating parameters such as module, pressure angle, addendum modification coefficient and rotational speed on these discharge surfaces and trapped volume is presented.