Friction loss at the piston ring-cylinder liner interface in an internal combustion engine strongly affects the fuel economy of automobiles. However, the relationships between viscosity characteristics of engine oils and friction at ring-liner interface are not well understood. In this study, we experimentally measured ring-liner friction using a floating liner method with various formulations of engine oils. Two types of engine oils were tested: Non-Newtonian oils that contain polymer additive viscosity modifiers (VMs) and Newtonian VM-free oils. We first tested VM-free oils with different base oil viscosities and found that the dominant friction energy mechanism changed from hydrodynamic lubrication to mixed lubrication as engine oil viscosity or piston speed were decreased. Friction energy reached a minimum at this transition point. We then tested VM-containing oil with different VM composition and found a relatively higher friction loss with VM-containing engine oils than with their VM-free counterparts in a mixed lubrication regime at the same low-shear viscosity. The relationships between effective viscosity and shear rate of VM-containing oils, measured through a high-shear viscometer, allowed us to estimate shear rate at the ring-liner interface. The most probable shear rate is consistent with the shear rate estimated from piston velocity and surface roughness, indicating that shear-thinning occurs at the ring-liner interface. These results suggest that shear-thinning can reduce fuel economy and durability in a mixed lubrication regime. Therefore, next-generation engine oils might need careful adjustment in viscosity and/or friction modifiers to improve their boundary friction performance.