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In this paper, data are presented showing how lubricant properties affect the heat flux, oil flow rates and temperatures within a turbocharged diesel passenger car engine. The oils tested cover a range of viscosities and base oil types. Mono-grades were used to remove the effect of shear thinning. The effect of viscosity modification was also examined. Lowest viscosity lubricants resulted in the lowest sump temperatures. More fuel was required to produce the same brake output from the engine with thicker oils. Engine oil heat rejection increased with viscosity, not just in absolute terms, but also as a fraction of the total heat loss. Viscosity does affect oil temperature through increased total heat from friction, and also through its effect on heat transfer. In addition, oil viscosity is itself dependent on operating temperature. Heat transfer theory suggests a relation between mass flow rate and heat transfer.