Today, the increasing use of oil and grease in industry with the aim of improving the performance of industrial machinery, equipment and protecting against damage to mechanical parts during work is not hidden from anyone. In this experimental study, the effects of gamma-rays on some physicochemical properties including the pour point, flash point, viscosity index, and kinematic viscosity of an engine oil at 40 °C, and 100 °C were investigated at various radiation doses up to 100 kGy. For this purpose, a commercial engine oil of type SAE 10W-40 was selected. To explore the structural changes of the samples at different doses, FTIR, and UV–Vis analyses were performed. These analyses confirmed the deformation of CH2 and CH3 groups in aldehydes at 2976-2838 cm−1, 1468 cm−1, and 1454 cm−1, and also predominance of the chain-scission or degradation up to 50 kGy. The findings indicated that within 0–50 kGy, the viscosity decreased significantly as the dose increased, while at 100 kGy, the hydrocarbons underwent crosslinking, leading to a negligible increase in the viscosity. Therefore, the stability of certain properties of the oil plays a crucial role in ensuring radiation tolerance designed for radiation applications.