The impacts of parameterized lateral ocean viscosity on climate are explored using three 120-year integrations of a fully coupled climate model. Reducing viscosity leads to a generally improved ocean circulation at the expense of increased numerical noise. Five domains are discussed in detail: the equatorial Pacific, where the emergence of tropical instability waves reduces the cold tongue bias; the Southern Ocean, where the Antarctic Circumpolar Current increases its kinetic energy but reduces its transport; the Arctic Ocean, where an improved representation of the Atlantic inflow leads to a better sea-ice distribution; the North Pacific, where the more realistic path of the Kuroshio leads to more realistic temperatures across the midlatitude Pacific; and the northern marginal seas, where stronger boundary currents lead to significantly less sea-ice. Although the ocean circulation and sea-ice distribution improve, the oceanic heat uptake, the poleward heat transport, and the large scale atmospheric circulation are not changed significantly. In particular, the improvements to the equatorial cold tongue did not lead to better representation of tropical precipitation or El Niño.