A viscosity index improver containing, in diluent oil, one or more optionally functionalized linear block copolymers having at least one block derived from alkenyl arene covalently linked to at least one block derived from diene in an amount that is greater than the critical overlap concentration (ch*), in mass %, for the linear block copolymers in the diluent oil; and ester base stock and/or at least one star (or radial) polymer, the star polymer being present in an amount such that the c/ch* value of the star polymer in the concentrate falls within the range of from 0.01 to about 1.6, wherein c is the concentration in mass % of star polymer in the concentrate and ch* is the critical overlap concentration in mass % for the star polymer in the diluent oil used to form the concentrate.