I recently watched some shows by Prof. Heinz Haber (1968) and I noticed, that he tends to append an "e" to the dative as in
auf dem Monde or
Udo Klinger lists the added "e" in parenthesis, which I interpret as "okay, but not necessary".
I suppose it is not okay to add an "e" if the noun ends with a vowel. But I am not sure. What are the rules when an extra "e" is okay and when it is not.
In contrast to the already answered question:
I'd like to know when you can add an "e", not when you have to.