Hello
The answer is more simple then first assumed..
I save two version of the library
- MDL-Lib saved in the oldest "possible" format (eg. 2008a)
- SLX-Lib saved in a newer Version (eg. 2013a)
If both files are in the path, older Matlab version take the mdl version and the newer take the slx. It works quite good. the only issue is with version 2012a/b because they already use the slx version but miss some features we used, so on startup you get a bunch of warnings (which can be ignored because they don't effect functionality)