They are filled with hydraulic oil at assembly.
The data sheet that came with my Stipe shocks says to use:
Quote:
Full Synthetic Hydraulic oil 30 to 40 weight.
Only fill to within 1/16th inch of bottom of filler hole ...
Leave air to allow for expansion.
Reseal plug with SMALL amount of sealer.
End Quote
Thanks for the response Benson. My Stipe Shocks were on the car when I bought it and I didn't receive the data sheet.
Since I posted my question I found on The Ford Garage to use "modern premium ISO 150 synthetic hydraulic oil".
But my original questions were still not answered.