B21 Turbo-What is this knob?

Gentlemen, Can anyone tell me what this knurled knob beneath my intake manifold on my B21 Turbo is for? I like to know it's purpose before I actually turn it. If it is simply plugging a hole, what should go there? Thanks, as always.

I think its a fine idle adjustment screw.

New engine mounts and change the gearbox oil. A little tune up never hurts either (change/gap plugs, ign components, check timing etc)

And your recommendations on the best fluid for an M46?