Lubricant degradation can have a significant impact on the efficiency and reliability of mechanical systems. This study analyses the effects of field operation on the physicochemical properties and tribological performances of gear transmission oils. Viscosity and composition of fresh-new oils and field-collected ones are analysed and tribological tests on the High Frequency Reciprocating Rig (HFRR) and the Mini Traction Machine (MTM) are carried out, to identify potential differences. Results reveal changes in intrinsic properties and tribological behaviour. They show that the evolution of the oil is highly dependent on the system and operating conditions, results that differ from those traditionally observed with engine oils.