Oil is a good example of this type of fluid. High viscosity can interfere with free seal movement and cause seal face contact problems. Lowering the viscosity can often increase the seal face wear as there is not enough film thickness to keep the surfaces separated. You need a film thickness of at least one micron to keep the lapped seal faces separated. Film building liquids - Petroleum products will form a varnish when first heated and then gradually form a layer of coke as the temperature is elevated.
Hard water is another example of a film building fluid.
It is black or reddish in color and will be attracted by a magnet. This abrasive material will collect on the seal components and destroy the dynamic O-Ring as well as restrict the movement of the seal causing the lapped faces to open. Magnetite is a severe problem in new , hot water systems. The problem will lessen as the system ages and the protective film stabilizes. Liquids that crystallize - Sugar and salt solutions are two examples of these fluids.