Gas- liquid separation is a typical process in many applications. For instance, gas separation is critical for the proper operation of electrical submergible pumps in the oil and gas industry as the pumps' performance and lifetime are severely reduced when working with high gas/ oil ratios. Gas- liquid separators are installed in oil production wells to reduce the void fraction at the pump inlet. The inverted- shroud gravitational separator stands out due to its efficiencies higher than 97%. This separator performs the gas separation process in two stages. The first is a segregation process related to inversion from liquid to gas continuous flow, whereas the second stage is related to gas entrainment associated with kinetic- energy dissipation process. The latter is more complex to model in the vertical than in the inclined separator's position. Previous studies revealed that the liquid flow rate and separator's inclination are relevant parameters for the gas separation efficiency (GSE). However, studies regarding the effect of the liquid viscosity on GSE are scanty. We evaluate the influence of the liquid viscosity and separator's inclination on the GSE of an inverted- shroud separator (IS- separator) with water- air and oil- air mixtures. Efficiency maps for each inclination and empirical correlations to predict the GSE in the vertical inclination are proposed. New experimental data collected for several liquid flow rates and separator inclinations are offered in this study as a starting point to develop universal GSE maps. Different gas separation phenomena are observed depending on the flow pattern at the inner annular channel (IAC) of the separator and its inclination. The experiments conducted with the water- air mixture indicated turbulent flow, while the oil- air mixture revealed laminar flow for both inclined and vertical positions. The results suggest that the greater the liquid viscosity, the higher the GSE. The efficiency maps indicate that it is possible to reach total gas separation (TGS) for many experimental conditions. In practice, our approach proposes an alternative technology where the variables that influence the production well's dynamic fluid level are actively controlled. Therefore, the IS- separator can operate under operational field conditions similar to those tested on a laboratory scale. This fact makes the IS- separator a promising tool for industrial applications.