Is it possible to change the value that is required for a friendship? Normally, the value is 40 and I want to change it on to 20.

I know there must be a setting somewhere but not exactly sure where to find it. However, take a look in Retuner>settings>general>by XML>sims3.gameplay.socializing. There's a number of setting in there for socailizing and you may find something in there. Hopefully someone else can give you an exact path.

Unfortunately this is table based and Retuner can't get to it. You can find it in the Socalizing XML however.

What exactly do I have to change in the Socializing XML ? I am using S3pe and I can not find the appropriate lines . Is there a tutorial how to change table based Settings? ?

I figure you already made or gotten a Socializing _XML tuning mod. After you opened it in S3PE, right click on the Socializing _XML line and choose edit in notepad (or texteditor).

Within Notepad (or texteditor) search for (CTRL+F) this exact phrase <LTR> (as written), then scroll down a bit, you'll see the first tuning value for LTR that is stranger, scroll further down and you'll find <Name>Friend</Friend>, within this block you'll find these entries:
<ul class="quotelist"><li><Score>20</Score></li><li><Liking>40</Liking></li><li><StoryProgressionEntry>40</StoryProgressionEntry></li><li><StoryProgressionExit>20</StoryProgressionExit></li></ul>
These are the values you'll need to change. I am not particularly sure if changing the Friend block lets say to something like this:
<ul class="quotelist"><li><Score>10</Score></li><li><Liking>20</Liking></li><li><StoryProgressionEntry>20</StoryProgressionEntry></li><li><StoryProgressionExit>10</StoryProgressionExit></li></ul>
That you will need to change the lower steps (Acquaintance, Distant Friend), so they don't overlap each other. You also might need to change Good Friend to connect it with Friend.

Hopefully our coder can elaborate on the values more better, but that's the section in the Socializing _XML you'll need.

As to if there is a tutorial how to change this, well there are general tutorials how to make a tuning mod, but specific tuning is about how it feels in the game and if its something you really want. Basically a modders purview.

Many thanks for the explanation, it works perfectly.

Only to my own understanding: what is Scoring? Are this the speeds at which the friendship values change?

Where do you see Scoring or did you meant Score? Scoring is not used in the Socializing _XML.

I believe score is the latest lowest value in a particular LTR (long term relationship), same as StoryProgressionExit, if the value is lower than the score than your relationship drops one level down. So say your Sim has a friend and hasn't talk with him/her for awhile, LTR decay sets in when it reaches 19 (on default setting) than it will become a distant friend.

It depends if a player has StoryProgression enabled or disabled where the game looks at the tuning, enabled StoryProgressionEntry and StoryProgressionExit, if disabled score and liking.

Though that's my assumption and I could be totally wrong in it, so will leave it up to our coder to either confirm or smacking me on my head