Hydraulic preload Adjuster
We use shock oil. Ohlins 1309-01 in our case.
The amount is more difficult to say as the process of bleeding the air out is complicated. Sufficient oil to move the spring seat through the full range. Less than 400cc is easily enough.
The large O ring seal gets dirty and fails on those. Sometimes the pivot bolt backs out as well.
Never argue with stupid people, they will drag you down to their level and beat you with experience.