Adding alumina to the conventional soda lime silicate glass composition improves many properties, however, also increases the melting temperature. In the current paper, alumina doping of soda lime silicate glasses and its implications to high temperature viscosity are investigated in order to verify the linearity when replacing SiO₂ for Al₂O₃. An anomaly in the linearly increasing viscosity was found when the ratio CaO/Al₂O₃ is <1, which is explained by Ca²⁺ acting as an inhibitor for viscous flow. The Angell and Rao and the Waterton-MYEGA models show very similar results, even for the extrapolated Tg. Thermodynamic data extracted using the Ojovan method generally increases linearly with the Al₂O₃ content. The Doremus fragility index shows a deviation in the linearity while the Angell fragility index on the other hand shows a linear increase with the Al₂O₃ content.