Non-Newtonian fluids are increasingly being transported by a variety of vehicles. It has been observed that vehicles containing such fluids demonstrate behaviour which could be explained only by non-linearity characteristics of such fluids. In this paper small oscillations of non-Newtonian fluids in tanks are considered. A numerical method suitable for both Newtonian and non-Newtonian fluids is suggested. Examples of numerical simulations with emphasis on Bingham fluids are given to demonstrate effects of non-linear properties.