Rheo-optical studies on aligned and unaligned melts of a main chain thermotropic polyester with flexible spacers are discussed in terms of the predictions of the Ericksen-Leslie equations. Experiments show that the material exhibits two different shear-flow regimes depending on the temperature at which the experiments are performed. It is shown, using conoscopy, that the director is flow-aligning at temperatures close to TNIand nonflow-aligning close to a smectic to nematic (TSN) phase transition. The implications of these results are discussed in terms of the change in sign of one of the Leslie viscosity coefficients and interpreted in the framework of the Ericksen-Leslie Theory.