The problem I see is that limiting discussion to bio and veg related topics could make for a slow moving and dry forum, but it would be purer for it.
In my mind, there really is not a great deal to say about veg oil, and anyone interested enough can find what they want to know without asking anything, bar a few odd questions. ( I will do a filtering piece for the wiki, this weekend hopefully)
Bio is far more involved than veg, but most things have been covered time and again. If anyone posts a new processor build, or has a problem, there is never any shortage of people to answer, no matter how the forum is organised and anything new and exiting seems to be taken up and discussed quickly.
However, I do agree that having a section for car repairs etc, would dilute the forum, but it may help keep it moving.
Saying that, if I was in the position of admin, I would want to keep the topics limited as well.
I think there ought to be a members only area, maybe there already is, it wouldn't suprise me if i hadn't noticed it.