94 altima - someone explain this

When i pull off my spark plug caps the 3rd spark plug port is filled with oil. I have no idea where its coming from, I changed my valve cover gaskets twice so far and it still fills up with oil. where is it coming from also It almost sounds like it skips that spark plug when the cars running idle.
If someone could explain this i would appreciate it

sounds like a leaking gasket for the valve cover. If I am not mistaken it looks like a rubber strip with holes in it. They are cheap and are not hard to replace. Worth it.

you need to also replace your spark plug tube seals!!