Fork oil, 5wt or 7.5wt (8wt)
buy it from motorbike shops or online at CRC (which is where mine is from)
They don't use that much and I always flush with some before putting the last batch in.
I currently use this in 5 an 7.5 wt (I use 7.5wt for my marzocchi bombers forks)
You can blend them together to get anything in between as well.
http://www.chainreactioncycles.com/Mode ... delID=8399
EDIT you'll be wanting the 7.5wt oil for your setup.
and depending on what is in the forks, there are valve spring washers that will alter it's preload. Really you
should have the spring made stiffer with more washers, not sure if you normal MAG21 (1993) should have them as they have a stiffer spring to start with I think.
But they'll still work fine without it being altered.
The valve spring holds a disc of mylar (or similar) down and the oil need to get enough force to push against his to make the fork move. The dial at the top allow you some adjustment from the outside, the spacers just do it from the inside iirc.