Never use any sealant on a head gasket, it will cause premature failure. Proper sealant is already on it where needed (also don't buy one and let it sit around with the plastic seal sheet removed for long periods of time -- the sealant will set and cause failures. Leave the gasket sealed until installed).

Never use ANY sealant on rubber seals of any kind. The rubber is the seal, and the cure for a leak is to replace the seal. Valve cover gaskets have a limited life (three years or so, usually), and should always be replaced if the valve cover is removed. Proper installation is critical, else they leak instantly.

More than one engine has been destroyed by excess sealant getting into the oil passages and causing a lubrication failure (ditto for paper towling used to wipe off the oil filter fittings!).

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!