The article presents the results of combined tribological and physicochemical studies of engine oils used in Autosan Sancity M12LF city buses at various mileages exceeding the manufacturer’s recommended oil change interval (60,000 km). Semi-synthetic oil of ACEA E4, E7, SAE 10W-40 class was analyzed, with samples taken from five buses with different degrees of interval exceedance (− 19% to + 35%). To address outlier influence, both classical Pearson and a more robust Spearman’s rank correlations were applied. While Pearson correlation suggested a very strong relationship (r = − 0.9946) between sulfur content and wear scar diameter, Spearman’s robust method confirmed a strong but more reliable association (r = − 0.810), indicating that classical correlation was inflated by outliers. A potential relationship was demonstrated between the degree of oil change interval exceedance and degradation of phosphate anti-wear additives. Anti-wear additive degradation follows a non-linear pattern and accelerates with successive exceedances of the recommended change interval. A degradation thresholds were identified: oil degradation becomes more pronounced after exceeding the interval by 10,000–15,000 km, while exceeding by more than 15,000–20,000 km results in potentialy increased engine component wear and elevated failure risk. The content of abrasive metals (Fe, Cu) in used oils significantly increased compared to fresh oil, with particularly high iron concentration (131.67 ppm) observed in the sample with the greatest interval exceedance. After exceeding the interval by 20,000 km, phosphate anti-wear additive depletion was 73% higher than when exceeding by 5000 km. The research results provide important information for bus fleet operators, allowing more precise determination of engine oil change intervals, which translates into extended engine life and reduced operating costs.