The recirculation of gases from the crankcase and valvetrain can potentially lead to the entrainment of lubricant in the form of aerosols or mists. As boost pressures increase, the blow-by flow through both the crankcase and the valve cover increases. The resulting lubricant can then become part of the intake charge, potentially leading to fouling of intake components such as the intercooler and the turbocharger. The entrained aerosol which can contain the lubricant and soot may or may not have the same composition as the bulk lubricant. The complex aerodynamic processes that lead to entrainment can strip out heavy components or volatilize light components. Similarly, the physical size and numbers of aerosol particles can be dependent upon the lubricant formulation and engine speed and load. For instance, high rpm and load may increase not only the flow of gases but the amount of lubricant aerosol. In this study, the number, size distribution, composition, and morphology of entrained lubricant aerosol is examined on a medium-duty diesel engine operating at different speeds and loads. A unique sampling apparatus is described for sampling the aerosol in the same manner that it enters the intake. In addition, the performance of oil separators is examined. Results demonstrate that the size distribution changes with load, and contains both a sub-micron and super-micron component. The chemical composition of the aerosol varies depending on engine speed and load and oil separator used, while TEM results show that aerosol morphology changes with lubricant viscosity and also engine conditions.