FORD ESE-M4G195-C - Sealant, silicone rubber - heat resistant. MIL-A-46146 - Adhesives - sealants, silicone, RTV, noncorrosive (for with sensitive metals and equipment).
Rear main seals can be made of rubber or silicone, and they can wear out because of age, the rotational forces of the crankshaft, corrosion from road salt and other environmental factors.