Sounds like you need a new oil filler cap only.

When the gaskets disintegrate or get hardened they leak from the gasket area while driving.

Sometimes just taking the cap off to fill the oil distrurbs the gasket surface enough from the happy seat it had formed while stationary for all that time, then it leaks when you put it back on.

The fact that your rubber gasket is missing is a no brainer, you need a new filler cap and have to clean the oil as best as you can off your valve cover and exhaust manifold.