Although allowable amounts of glycol contamination in diesel engine oil, no research has been conducted on how these levels and varying loads affect engine performance. The research used a four-stroke diesel engine to investigate the effect of different glycol contamination levels (0, 120, and 220 ppm) under two engine loads (4.5 and 9 kW). Brake specific fuel consumption, brake thermal efficiency, friction power, and exhaust gas temperature were measured to determine the engine performance. The experiment used the factorial arrangement in a completely randomized design (CRD) with three replicates. Increasing the contamination levels from 0 to 120 and then to 220 ppm under constant engine load significantly increased brake specific fuel consumption, friction power, and exhaust gas temperature and decreased brake thermal efficiency. Increasing the engine load from 4.5 to 9 kW with constant oil glycol contamination levels significantly increased brake thermal efficiency, friction power, and exhaust gas temperature and decreased brake specific fuel consumption. The results of the statistical analysis showed significant differences in the interaction between glycol contamination levels and load levels. Lower friction power (5.6 kW) and exhaust gas temperature (165.33 °C) were produced by combining the first contamination level (0 ppm) with the first load (4.5 kW), while the lowest brake specific fuel consumption (0.24 kg/kWh) and highest brake thermal efficiency (32.14%) were produced by combining the first contamination level (0 ppm) with the second load (9 kW). The study concluded that diesel engine performance decreases when engine oil is exposed to the permissible levels of contamination above with changes in engine load. This study can aid diesel engine maintenance and operational instructions, particularly in glycol-contaminated diesel engine oil.