Bad valve cover gaskets are a relatively mild problem for car owners that if left unchecked can balloon into engine reconstruction. A faulty cap may cause oil levels within the vehicle's engine to diminish rapidly, potentially resulting in the drive train throwing a piston or at worst an engine crack. A careful driver should identify which symptoms their vehicle is experiencing, and see a mechanic before the problem becomes too expensive to tackle.
An oil leak may result from a cracked or otherwise damaged valve cover gasket. A leak will appear as drops or a puddle of oil underneath the vehicle when it is stopped and turned off. A leak can dramatically effect oil levels and should be dealt with by replacing the valve cover as soon as possible.
Different from a leak, oil seepage is quite common for vehicles with over 30,000 miles of drive time. A small seep from the valve gasket will attract dust or grit that can give away the problem as a seep over an outright leakage problem. While a nuisance, it is not necessary to replace the valve gasket that is experiencing seepage. The condition can be monitored and if it turns into a leak then the gasket should be replaced.
Oil that is thrown from the engine via a faulty valve gasket while driving can be burnt off by engine heat. Gray smoke from the vehicle's hood while idling may be a sign that the valve gasket is either cracked, loose or missing entirely. Since this can effect oil levels and the engine's ability to operate, it should be turned off immediately and checked by a professional mechanic to have the part replaced and the engine checked for damage.