An improved viscosity index improver and an oil composition containing such a viscosity index improver are disclosed. The viscosity index improver is a carboxyl functionalized block copolymer which comprises of at least one monoalkenyl aromatic hydrocarbon polymer block and at least one conjugated diolefin polymer block. The oil can be selected from the group consisting of crude oils, mineral and synthetic lubricating oils, fuel oils, diesel oils, hydraulic oils, automatic transmission oils, and mixtures thereof.