Automotive Sealants
There are three types of sealants manufacturers commonly use to assemble their automotive engines. These are RTV or room temperature vulcanizing sealant, anaerobic sealant, and pipe joint compound. Of the three, pliable silicone sealants like RTV are most common when repairing or rebuilding an engine. This is due to engine vibrations and imperfect surfaces on engine parts. RTV sealant is not to be used in areas with high temperatures like exhaust manifolds. Always use the appropriate sealer recommended by the engine manufacturer for the specific job. 
Silicon based aerobic sealants like RTV or room temperature vulcanizing are used throughout the engine sealing all kinds of leaks. If you've been a technician for any length of time, you've used RTV. An aerobic sealant dries in the presence of air and forms a flexible rubber gasket. This sealant along with the other three mentioned must be applied to a clean surface. Scrape any old gasket of the surface with a gasket remover and clean the surface thoroughly for a good leak proof seal. Always follow manufacturers specifications and procedures when using any sealant with a sensor or actuator. This sealant may be used in place of a conventional gasket only in specific applications. Sometimes Teflon tape is a great option to help seal threaded parts.
Anaerobic sealants are used on machined surfaces. These sealants dries and hardens in the absence of air, this is the opposite of aerobic sealant that dries in the presence of air. These sealants are capable of withstanding temperatures in excess of 300 F. These sealants are more likely to be found inside of an engine.
Keep these sealants out of bolt holes, they will fill the threaded hole and effect the torque readings. Like the oil pan in the illustration spread the sealant around the inside of any bolt holes facing the fluid and the inside of the engine.
