Literally, the valve cover serves as the covering for the valve trains. This is removed to adjust the valves. Oil is pumped up through the pushrods and dispersed underneath the valve cover, which keeps the rocker arms lubricated. Holes are in many locations in the engine head, so that the oil recirculates back down the oil pan. Thus, the valve cover must be oil-tight. It is often the source of leaks. Some valve covers are distorted on older cars because the valve cover is made of very thin sheet of metal and can't withstand over-tightened screws.
With this oil-leaking problem, you will need a valve cover gasket. It is a simple gasket that helps keep leaks, oil, water and other debris out of the cylinder head and engine. It is but common for a valve cover gasket of an older engine to develop a leak. This does not pose a serious problem. Its replacement is a minor job, but it should be immediately attended to.
In modern machinery, more sophisticated types of valves are being produced to answer the demands of oil leakage in the upper part of the engines. Some could be made of thermoplastic, sprayed with chrome, aluminum or carbon fiber. Chrome valve covers abound in most street and performance cars of today.