In this study, the evolution of thermophysical and rheological properties of Aeroshell Turbine Oil 500 during its service life was experimentally investigated in two helicopter platforms operating under different conditions: a Military Attack Helicopter (MAH) subjected to severe thermal and mechanical loads, and a Military Utility Helicopter (MUH) operating under comparatively milder conditions. Engine and gearbox oil samples were collected over 200 hours of operation at 25-hour intervals. The experimental analysis included measurements of viscosity, thermal conductivity, density, and specific heat capacity, along with the evaluation of thermal diffusivity and rheological behavior. The results indicated that oil degradation in the engine was significantly more severe than in the gearbox. Kinematic viscosity increased by 34% in the MAH engine and 25% in the MUH, leading to a proportional rise in frictional losses. Thermal conductivity decreased by 23.7% in the MAH and 18.9% in the MUH, reflecting a reduction in oil cooling capability. In both platforms, specific heat decreased while density increased, indicating structural changes associated with thermal aging. Rheological analysis showed Newtonian behavior in the gearboxes of both helicopters. However, engine oil exhibited nearly Newtonian behavior after prolonged operation and transitioned to mild shear-thinning behavior in the MAH engine during the final service stage. These findings highlight the importance of continuous monitoring of oil thermophysical properties in high-load aircraft systems and provide a basis for optimizing maintenance strategies, lubricant selection, and thermal management system design.