Heat can injure some carbon faces by melting the fillers and expanding the air pockets trapped below the surface, causing pits in the carbon that will prevent it from passing a fugitive emission test. Plated surfaces can heat check and crack due to the differential expansion between the coating and the base metal. Many products can vaporize at elevated temperature, blowing the faces open and leaving solids between the lapped faces. Heat will change the viscosity of many liquids. It many cases it will diminish, but in some cases the viscosity can increase.
Corrosion always increases with additional heat.
Petroleum base products can coke between the faces. Stick with hydraulically balanced seals. The unbalanced versions do not make any sense. In addition to low heat generation. The O-ring version will seal either vacuum or pressure.