Deposit formation within turbocharger compressor housings can lead to compressor efficiency degradation. This loss of turbo efficiency may degrade fuel economy and increase CO2 and NOx emissions. To understand the role that engine oil composition and formulation play in deposit formation, five different lubricants were run in a fired engine test while monitoring turbocharger compressor efficiency over time. Base stock group, additive package, and viscosity modifier treat rate were varied in the lubricants tested. After each test was completed the turbocharger compressor cover and back plate deposits were characterized.A laboratory oil mist coking rig has also been constructed, which generated deposits having the same characteristics as those from the engine tests. By analyzing results from both lab and engine tests, correlations between deposit characteristics and their effect on compressor efficiency were observed. The physical characteristics of these deposits, as well as parameters affecting deposit formation such as the chemistry of the oil formulations, oil aerosol particle sizes, and mass of oil mist flow are discussed. The rough/smooth and dry/wet qualities of the deposits were found to correlate most with compressor efficiency loss; thickness and mass of deposits did not correlate.