Dear all, has anyone had experience extending thier L visa ? Which documents are required now?

Is it 30 days ? Can it be extended twice like M type?
I have also heard that getting a L type ,might make it harder to get a M type in the future, has anybody heard it or experienced it?
I always get M type, but now our embassy is requesting the dully Authorized letter,which is hard to get & usually denied ..that's why Im thinking of getting the L type now so that I could have time to find a way to get that authorized letter.
Thank you