It has been frequently hypothesized that there is an “effective immobile layer” attached at the solid–liquid interface to represent the hindering effect because of the complicated composition on the flow of crude oil in nanopores. Nevertheless, the resulting viscosity discontinuity is physically problematic, and the effect of viscosity transition was not incorporated. In this paper, based on the reduced form of the continuous viscosity profile, the numerical and analytical models for reduced velocity profiles (quantifying the magnitude and the shape) and the reduced pore radius (the ratio of equivalent and actual pore radii) are obtained and compared with each other, respectively. The reduced pore radius establishes a link between the “effective immobile-layer” simplification and the viscosity transition. Detailed sensitivity analysis is conducted to study the impact of viscosity transition (indicated by the curvature constant and the viscosity ratio) on the reduced viscosity profile, reduced velocity profiles, and the reduced pore radius, separately. Results show the microscale flow patterns that cannot be reflected by the existing body of methodology. This work is important for understanding the flow characteristics of crude oil in shale and tight rocks, where nanopores are dominating and the effect of the interfacial viscosity transition can be significant.