To follow Stack Overflow convention I suggest to associate f# to lang-ml syntax highlighting.
lang-ml already does this for F# as written on the main site. Syntax highlighting appears to be identical with usage on Stack Overflow:
Usage on Programmers in a preview box:
The raw markdown is an identical copy-paste. I also tested it on Arqade where the syntax is not enabled as well as Game Dev where it is fully enabled. Associations appear to be universal where syntax highlighting is enabled. This makes sense because Stack Exchange outsources the highlighting.
It doesn't work on Meta but no syntax highlighting seems to.