Viscosity index improver additives for lubricating oils and process for their preparation

Viscosity index improver (V.I.I.) additive for lubricating oils comprising an ethylene-propylene copolymer, said additive having the following features: - a shear stability index (SSI) comprised between 18% and 30%, measured according to ASTM D7109-12 standard (30 cycles of shearing) in Group I base oil; - a thickening power (TP) comprised between 3 cSt and 8 cSt, measured according to ASTM D7042-04 standard in Group I base oil; - a gelation index defined by the ratio: ([eta]0 DEG C/[eta]40 DEG C)/([eta]60 DEG C/[eta]40 DEG C) where [eta] represents the steady state dynamic viscosity measured in Group III base oil, said gelation index being lower than or equal to 7000; - an incipient gelation temperature, measured in Group III base oil, at a temperature lower than or equal to 10 DEG C; - a shape stability defined according to the following formula: [Delta]L% = [(L/L0)-1)]*100 wherein L is the length after 24 hours and L0 is the initial length equal to 10 cm, lower than or equal to 5%.