Due to increasing demands for further CO2 reduction and tighter exhaust emissions regulations, automakers are increasingly downsizing turbo-charged diesel engines by raising specific power, or adopting low-pressure loop exhaust gas recirculation (LPL-EGR) systems to improve the EGR rate.However, adopting a higher boost pressure to increase the specific power, or introducing hot exhaust gas before the turbocharger compressor with the LPL-EGR system creates higher gas temperatures in the compressor, which results in soot-containing deposits derived from the engine oil in the compressor. This phenomenon causes significant deterioration of turbocharger efficiency. Therefore, countermeasures such as restricting boost pressure or limiting EGR usage in the operational map are necessary to prevent engine performance deterioration. Increasing the gas temperature in the compressor while preventing deposit formation should enable further improvements in fuel consumption and engine power.This paper investigates the root causes of compressor deposit formation to achieve better engine performance. Fired diesel engine tests were run at various operational conditions to investigate the influence of engine oil degradation and oil properties. Deposits from the compressor were recovered and analyzed in detail. As a result, it was confirmed that the evaporation characteristics of engine oil have a significant influence on deposit formation, and that engine oil can be formulated to provide better deposit control at greatly higher gas temperatures in the compressor while maintaining other engine oil properties suitable for production.These results should contribute to further improvements in fuel consumption and power for future diesel engines.