Dilute solutions Of a styrene-hydrogenated butadiene copolymer, a viscosity index improver, were studied by static and dynamic light-scattering techniques. In n-hexane (a model solvent for paraffinic oils) and a mineral base oil, aggregation is observed below 30-degrees-C. In cyclohexane (a model solvent for napthenic oils) only isolated polymers are present in the whole temperature range. Kinematic viscometric measurements from 20 to 60-degrees-C in the mineral oil show a continuous increase of the intrinsic viscosity together with a decrease of k(H), the Huggins coefficient, from 2.5 to 0.5. At shear rates between 5000 and 40000 s-1, a large shear thinning is observed at room temperature for the polymer in the mineral base oil. This effect progressively disappears as the temperature increases and the suspension becomes Newtonian near 100-degrees-C. All results can be interpreted in terms of micelle formation.