This paper describes the observed impact of aniline octane booster and more specifically N-Methyl Aniline (NMA) on lubricating oil, following field issues encountered in vehicles in certain areas of the world where aniline based octane booster was assumed to be used. The observed field issue was heavy sludge formation, leading to engine malfunctions.The impact of NMA on lubricating oil could first be replicated using a modified version of the CEC L-109 oxidation bench test, with oil diluted with fuel and NMA at start of test. Significant kinematic viscosity (KV100) increase at the end of test was evidenced, especially as NMA content was increased in the oil. Total base number (TBN) evaluated at end of test also showed values above fresh oil (at or above 4.5 wt% NMA) that was consistent with observations from the field.The assessment of NMA impact was then scaled up using ACEA black sludge engine test. Tests run with 3.5 wt% NMA added to black sludge reference fuel led to poorer sludge rating compared with standard black sludge test. Increased sludge formation in presence of aniline based octane booster evidenced on black sludge test was also consistent with observations from the field.Modified CEC L-109 oxidation bench test correlated to black sludge test in terms of sludge rating trend. However, no correlation could be found for kinematic viscosity increase and further work is in progress to understand the root cause, including impact of engine oil formulation.