[Ben006]

Very common issue indeed !

-Change fluid and maybe add some additive, I think there are better solutions out there than the Porsche oil (especially in terms of price...).
-Check mounts : Yes, but not likely to be a real problem for your concern.
-G-box detent : Yes ! I haven't seen a single bad review. Though... I haven't been able to install mine because I can't get the old one out !
-Check shifting cables and shifter mechanisme, too much play could result in a bad gear engagement.