There are pros and cons to using either cork or rubber for your valve cover gasket. Cork is a natural material that is very compressible, meaning it can conform to irregularities in the surface it is sealing against. This can make for a better seal, particularly on older engines where the mating surfaces may not be perfectly flat. Cork is also resistant to oil and chemicals, making it a good choice for use in an engine environment. However, cork can also be difficult to work with and install, particularly if you are not experienced in doing so. It can also be subject to shrinkage and deterioration over time, which can eventually lead to leaks. Rubber is much easier to work with and install, and is also more resistant to shrinkage and deterioration. However, it is not as compressible as cork and may not seal as well against irregularities in the surface. In general, either cork or rubber can be used for a valve cover gasket. If you are experienced in working with cork, it may provide a better seal. If you are not experienced in working with cork, rubber may be easier to install and less likely to leak over time.