Slug frequency is a key parameter in the modeling and flow assurance management of gas–liquid slug flow encountered in the petroleum industry. While slug frequency has been extensively investigated in horizontal and vertical pipes, its behavior in inclined flows with high-viscosity liquids remains less documented. In this short communication, experimental datasets reported in the literature are re-analyzed to investigate the combined effects of pipe inclination and liquid viscosity on slug frequency. The results show that, for a given liquid viscosity, slug frequency increases with pipe inclination. In contrast, the influence of liquid viscosity depends largely on the regime, showing opposite trends in flows dominated by gravity and inertia. The transition between these regimes is observed at a mixture Froude number of approximately 1.2. Further experimental investigations of slug frequency in inclined gas–liquid flows with high-viscosity liquids are required to better explain this behavior and to develop more robust slug frequency models for gas-high-viscosity liquid two-phase flow in inclined pipes.