Process for improving the viscosity index of lubricating oil compositions by adding a triblock copolymer of general formula (Ib) A-B-A (Ib), wherein each segment A is prepared from a monomer mixture comprising: 80 to 100% by weight of monomers being selected from the group consisting of C 1-4 -alkyl (meth)acrylates, styrene, benzyl (meth)acrylate and mixtures thereof, preferably C 1-4 -alkyl (meth)acrylates; and 0 to 20% by weight of C 5-32 -alkyl (meth)acrylates, preferably C 10-16 -alkyl (meth)acrylates, based on the total weight of segments A, and segment B is prepared from a monomer mixture comprising: 80 to 100% by weight of monomers being selected from the group consisting of C 5-32 -alkyl (meth)acrylates, preferably C 10-16 -alkyl (meth)acrylates; and 0 to 20% by weight of monomers being selected from the group consisting of C 1-4 -alkyl (meth)acrylates, styrene, benzyl (meth)acrylate and mixtures thereof, preferably C 1-4 -alkyl (meth)acrylates, based on the total weight of segments B, characterized in that the weight-average molecular weight of the triblock copolymer is in the range of 3,000 to 30,000 g/mol, preferably 3,500 to 25,000 g/mol, more preferably 4,000 to 20,000 g/mol, determined by gel permeation chromatography against PMMA standards, characterized in that the triblock copolymer of general formula (Ib) consists of: (a) 45 to 70% by weight of segments A and (b) 30 to 55% by weight of segment B, based on the total composition of the triblock copolymer. wherein each segment A is prepared from a monomer mixture comprising: 80 to 100% by weight of monomers being selected from the group consisting of C 1-4 -alkyl (meth)acrylates, styrene, benzyl (meth)acrylate and mixtures thereof, preferably C 1-4 -alkyl (meth)acrylates; and 0 to 20% by weight of C 5-32 -alkyl (meth)acrylates, preferably C 10-16 -alkyl (meth)acrylates, based on the total weight of segments A, and 80 to 100% by weight of monomers being selected from the group consisting of C 1-4 -alkyl (meth)acrylates, styrene, benzyl (meth)acrylate and mixtures thereof, preferably C 1-4 -alkyl (meth)acrylates; and 0 to 20% by weight of C 5-32 -alkyl (meth)acrylates, preferably C 10-16 -alkyl (meth)acrylates, segment B is prepared from a monomer mixture comprising: 80 to 100% by weight of monomers being selected from the group consisting of C 5-32 -alkyl (meth)acrylates, preferably C 10-16 -alkyl (meth)acrylates; and 0 to 20% by weight of monomers being selected from the group consisting of C 1-4 -alkyl (meth)acrylates, styrene, benzyl (meth)acrylate and mixtures thereof, preferably C 1-4 -alkyl (meth)acrylates, based on the total weight of segments B, 80 to 100% by weight of monomers being selected from the group consisting of C 5-32 -alkyl (meth)acrylates, preferably C 10-16 -alkyl (meth)acrylates; and 0 to 20% by weight of monomers being selected from the group consisting of C 1-4 -alkyl (meth)acrylates, styrene, benzyl (meth)acrylate and mixtures thereof, preferably C 1-4 -alkyl (meth)acrylates, characterized in that the weight-average molecular weight of the triblock copolymer is in the range of 3,000 to 30,000 g/mol, preferably 3,500 to 25,000 g/mol, more preferably 4,000 to 20,000 g/mol, determined by gel permeation chromatography against PMMA standards, characterized in that the triblock copolymer of general formula (Ib) consists of: (a) 45 to 70% by weight of segments A and (b) 30 to 55% by weight of segment B, (a) 45 to 70% by weight of segments A and (b) 30 to 55% by weight of segment B,