The effect of downward and upward pipe inclinations on flow characteristics for high viscosity oil-gas two-phase flow was experimentally studied. 400 experimental tests were conducted in a 50.8-mm ID pipe for ±2° inclinations. Four different oil viscosities, namely, 0.585, 0.378, 0.257 and 0.181 Pas, were considered. Superficial gas and liquid velocity varied from 0.1 to 3.5 m/s and 0.1 to 0.8 m/s, respectively. Flow pattern and pressure drop are reported. The experimental results were used to evaluate different flow pattern maps, models and two-phase flow correlations.