Driven by applications using herringbone grooved bearings lubricated with a gas operating under conditions where the perfect gas assumption is not valid, the real gas effect has been introduced into the existing narrow groove theory. The enhanced model has been linked to a rotordynamic code and to a multi-objective optimizer resulting in optimum bearing geometries that differ from the ones under the perfect gas assumption. It is suggested that rotors on bearings that have been optimized under the perfect gas assumption may get unstable if they are submitted to a lubricant that operates close to the saturation line. (C) 2010 Elsevier Ltd. All rights reserved.