In this work a modification of Cox's model for the deformation of a highly viscous drop is presented, which describes accurately the period of the oscillations during transient deformations. It is demonstrated that the oscillation period depends not only on the shear rate as predicted by Cox model but also on the viscosity ratio. This modified model is obtained from the analysis of numerical simulations of drop deformation using the boundary element method, while the experimental results are those obtained in quasi-simple shear flow generated in a two roll mill (2TRMs) device, as well as experimental data available in the literature. The predictions of the periods match very well with experimental data. (C) 2018 Elsevier Ltd. All rights reserved.