The importance of the Chevrolet valve cover gaskets and seals cannot be understated. Not only do they keep the oil inside of the engine, but since this part is on top of the engine it also helps to keep the entire engine clean and free of oil and dirt; this includes all of the delicate sensors. On a 4 and 6 inline cylinder engine there is only one set of valve cover gasket and seals. If you have a V6 or V8 engine then there will be 2 valve cover gaskets and seals. The polymer gasket on your Chevrolet engine will wear out in time due to the extreme temperature it is exposed too making replacement recommended. Any Chevrolet owner can inspect the top and sides of their engine to see if oil is dripping down the side of the head or block. If so, it is time for replacement. Some manufacturers recommend service about every 50,000 miles. The Chevrolet valve cover gaskets and seals of today are different for the newer engines than the older models. No longer is a cork gasket used but a polymer one. This extends the life of the seal for more control over where the oil will end up. Immaterial of the material your valve cover gasket and seals are made from, the old ones have to be completely removed from all surfaces before the new ones are put into place. If this is not done, a leak will occur where part of the old gasket remains. The replacement of the valve cover gaskets and seals can be done by the Chevrolet owner. One word of caution when undertaking this procedure is not to over tighten the valve cover bolts. This would smash the gasket and can cause it too split where oil can leak out.