I simply put quite some grease on both sides of the cork before assembly, the grease will keep the oil in.

Works fine for me and easy removed when needed and no meshing with silicon or other stuff.

My WDG3L chaincase does have oil and amazingly does not leak! Many years ago, I fitted the "T" section rubber gasket mod in place of the original round one and backed that up by careful attention to the spacer for the through-bolt, which I understand is where most of the problem originates for the AMC tin chaincase........the spacer gets left out, or is the wrong one, and the case distorts.