Lung surfactant (LS) is a complex mixture of lipids and proteins originating from the type II cells that line the alveolar epithelial walls. LS reduces surface tension in the alveolar spaces, which minimizes the work of breathing and prevents the alveolar collapse. A lack of functional surfactant due to premature birth leads to neonatal Respiratory Distress Syndrome (nRDS) which is routinely treated in developed countries with animal derived replacement surfactant. One of the essential features of good LS is to reduce the surface tension at the alveolus air-water interface to near zero. Presently, there is no simple theory relating chemical or physical properties of a monolayer with its ability to lower surface tension in the dynamic process of breathing. Monitoring the viscoelastic properties of interfacial films allows hypothesizing how surface viscosity depends on lipid and protein composition, packing properties and other variables at different states of the breathing process. Our viscometer consists of a Langmuir trough equipped with Helmholz coils which generate a controlled magnetic force to move a magnetic needle floating on the monolayer. If the applied force is oscillatatory, the complex shear modulus is determined from measurements of the in-phase and out-phase of the resulting strain. If the force is constant, the shear viscosity can be extracted from the terminal velocity of the needle. Here we present the behavior of LS monolayer concerning surface viscosity at different surface pressure, composition and phase in order to make a prediction about the optimal composition of LS.