A process is described for obtaining viscosity index improver compounds (V.I.I.) having an improved dimensional stability and capable of enhancing the low temperature rheology of lubricating oils, the above process comprising the treatment of amorphous EP(D)M copolymers or terpolymers, optionally mixed with semi-crystalline EP(D)M, mixed with linear polyethylenes, the treatment being effected in the presence of one or more substances of a hydroperoxide nature and optionally in the presence of small quantities of polyfunctional vinyl monomers, the above treatment being carried out under high shear conditions at a temperature ranging from 75° C. to 260° C., preferably from 140° C. to 210° C.