Polymer additives, named Viscosity Modifiers (VMs), have been added to motor engine lubricants for more than 50 years, where they are used to increase the viscosity index of their blends. It is well known that solutions of VMs exhibit shear thinning under high strain rate conditions as are present in engine journal bearings. In the past, this shear thinning was regarded as undesirable since it might reduce hydrodynamic film thickness. However it has been reported that multigrade oils containing VMs provide lower engine friction than polymer-free oils of comparable viscosity and the most likely origin of this is a reduction of hydrodynamic friction due to temporary shear thinning of the polymer solutions. In this thesis, the role of polymer shear thinning in reducing hydrodynamic friction is clarified by the experimental work carried out on a range of different polymer types. This study includes the viscosity measurements over a wide range of shear rate and also friction and film thickness analyses performed in a soft elastohydrodynamic lubrication contact in order to avoid piezoviscous effects. A very recent technique based on laser induced fluorescence technology is used to evaluate the film thickness. These experimental results are compared with theoretical calculations, which allows based on the rheological data of each polymer solution, the impact of polymer shear thinning on hydrodynamic friction to be quantified. The studied polymer solutions show a decrease in film thickness and in hydrodynamic friction compared to Newtonian fluids of similar viscosity. This reduction is ascribed to the temporary shear thinning of polymer blends. The proposed theoretical models show good agreement with the experimental values. They appear to be useful tools to quantify the impact of polymer shear thinning on film thickness and friction. In addition to this main work, the polymer solutions are analysed under high contact pressure and the shear stability of some commercials viscosity modifiers are studied. A new method is developed to quantify the permanent viscosity loss of each polymer solution and this is compared to the industrial Kurt-Orbahn test.