This paper proposes a semi-flooded cooling concept for a high speed machine design. The electrical machine considered is for an aircraft starter-generator system where power density and efficiency are crucial. In order to achieve the high power density permanent magnet machine, a direct oil cooling strategy for the machine stator is adopted. A stator oil sleeve is designed to separate the machine into an oil-flooded stator chamber and an oil-free rotor chamber to avoid windage loss due to the high rotor speed. Different materials, Carbon Fiber (CF) and E-glass fiber (GF) are considered for the oil-sleeve design. Implications of the aforementioned materials on the structural design are investigated and optimized accordingly using finite element methods. It is shown that the oil sleeve made of CF provides structural capability with a minimum sleeve thickness. However the sleeve made from GF has the benefit of having no electrical conductivity. Both oil sleeves are made and the GF sleeve is successfully applied to the machine and tested.