Piston-ring packs are major components of automotive engines. These engines are commonly lubricated with multigrade oils, which usually exhibit a non-Newtonian (shear-thinning) behaviour. That is, their viscosity decreases with an increase in shear rate. Consequently, this will effect the tribological performance of piston-ring packs. In this work, a two-space-dimensional lubrication model of piston-ring packs developed initially for Newtonian lubricants has been extended to include the effect of lubricant shear-thinning. A modified power-law model was used to characterise the lubricant shear-thinning behaviour, and it was incorporated in an existing flow-continuity algorithm, which forms the base of the current ring pack lubrication model. The extended lubrication model has been verified with published experimental results. Good agreement between theory and experiment has been achieved in terms of friction mean effective pressure (fmep). The effect of lubricant shear-thinning on the performance of a modern automotive piston-ring pack was examined. Both circular and non-circular cylinder bores were used to conduct the examination. Results show a significant lubricant shear-thinning effect.