Viscosity is the most important physical feature of the lubricants for it varies with the temperature and the shear rate and determines their performance. Multi-grade oils contain organic polymer additives to improve the viscosity index. The organic polymers are base oil soluble, have molar mass from 104 to 106 g/mol and are able to reduce the dynamic viscosity’s dependence on the temperature. The viscosity index additives are those polymers whose chains may be blocked by twisting them either in between or with the oil’s, which alters the speed, the geometry and the molecular movement of the both components and varies along with the temperature. The chains get blocked by twisting and therefore cannot move easily at high temperature as compared to no twisting and thus we have a loss in viscosity. In this paper I undertook to determine the rheological characteristics of the copolymer hydrogenated poly (isoprene-co-styrene) solutions. I have studied the 3, 6, 10, and 12% concentrated solutions for a temperature interval from 40°C and 90°C in 10 degree steps, and for the shear rate from 3 to 1312 s-1 in the second domain. The measurements were performed using a Haake VT 550 rotational rheometer with a HV1 sensor. The rheology of the solutions has been impacted by the shear rate, the temperature and the concentration of the copolymer hydrogenated poly (isoprene-co-styrene). I have found two models that successfully describe the rheological behaviour of the solutions compared to other models that do not consider the copolymer’s concentration.