The objective of this work is to quantify the effect of shear rate on the viscosity of multigrade lubricants. The study is based on the assumption that multigrade lubricants are two phase systems with the phases responding more-or-less independently to external mechanical actions. The assumption generates two questions: 1) Are there rules of behavior respected by all members of a given family of blends (same additive in different concentrations and the same class of base oils with different viscosities)? and 2) To what extent do the nature of the additive and the base oil affect these rules?