If the B310 strut is stock you can service them. I don't think you can get any of the internal parts though.

Yes, you can adjust the damping to some degree by changing the oil in the strut. Somewhere in the archives is the spec on how much oil to put in the strut. I have done this and it works pretty well for street applications. Won't go far enough for serious autocross or track use though.

The strut insert is a cartridge that replaces the entire internals of the strut. To use these you unscrew the existing top, dump the contents out, drop the insert in the empty strut tube, and screw on the cap that comes with the insert.

If you want to change the oil in your existing struts you unscrew the gland nut on the top of the strut and remove it. Next, you will need to remove a small O ring that is on top of the top of the internals. It is partially trapped in a groove and can be hard to remove without damaging. I found a dental pick to be usefull here. After the O ring is out you can gently lift out the strut shaft. There are 4 main pieces to the strut damper assembly-
The main strut body,
The top bearing that the gland nut presses down,
The piston and shaft assembly,
The tube and foot valve that the piston runs in.

Be careful as when you pull up on the shaft as all the oil on top of the piston will come out of the top of the strut.

Dump all the oil into a container so you can measure how much to put back in.

When I did my quickie strut stiffening exersize I used 20wt motor cycle fork oil. It firmed up the old struts quite nicely.

This would also work on the 1200 stock struts assuming that they were not shot or leaking...