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. If the crystals form between the faces they can destroy the carbon. If they form in the sliding or flexing components they will open the seal faces as the shaft moves. Any leakage across the seal faces will form a solids build up on the other side of the seal causing interference as the seal tries to move when it compensates for wear.