Why do you need a quality valve cover gasket
Valve covers are typically made of stamped steel, lightweight aluminum or magnesium and may warp over time, so the sealing surface that mates with the cylinder head is no longer flat, creating oil leaks. The lightweight material can be deformed by excessive torque on the fasteners holding it in place. Overtightening the valve cover fasteners may crush the gasket and damage the valve cover creating further leakage.
Rubber gaskets may develop a compression set over time and will no longer be able to effectively seal the joint between the cover and head. Older applications using cork-rubber gaskets may develop compression set or become saturated over time again leaking onto the cylinder heads.So it is important to install a quality valve cover gasket