Hydraulic oils in mobile machinery face intense mechanical and thermal stresses, which accelerate their degradation. This study provides a side by side field comparison of how VG46 and VG68 hydraulic oils age in sugarcane harvesters working in the hot, dusty climate of Khuzestan, Iran. A continuous 1,000 hour field test was conducted, with oil samples collected every 100 hours in accordance with ISO standards. Key properties-including rheological characteristics, chemical stability, and particle contamination levels-were analyzed (following ISO and ASTM methods) to assess degradation under real operating conditions. The results show that kinematic viscosity at 40°C decreased by 23.2% for VG46 but only 12.1% for VG68, indicating that mechanical shear degradation dominated over thermal oxidation. The viscosity index dropped by 16% for VG46 versus 10% for VG68, confirming mechanical scission of polymer chains in the viscosity index improvers. Chemically, VG46 exhibited a higher oxidation rate, with a sharp increase in total acid number between 300 and 400 hours—a clear sign of premature antioxidant depletion. Regarding contamination, VG46 reached a final cleanliness code of 24/24/23 (NAS Class >12), whereas VG68 maintained a superior code of 23/22/21 (NAS Class 12). These findings suggest that VG68 offers better rheological and tribological stability under severe duty cycles. Therefore, using VG68 hydraulic oil can significantly reduce the risk of sudden component failures and extend the service life of hydraulic power transmission systems operating in harsh environments.