I believe Mainship abandoned Hynautic before the 390 went into production, but I could be wrong. Most 390's have Sea Star by what was Teleflex, which is a simple and reliable system.
Picture below is a Hynautic Reservoir, if you've seen one of these somewhere aboard check the pressure guage.
If you have a Sea Star system (black helm) use only Sea Star fluid available at most marine stores. Others will say there are cheaper substitutes but I consider this a bad place to save a few dollars.
As Teleflex hydraulic is now Sea Star Solutions and Teleflex bought Hynautic, manuals for both systems can be found here:
This should get you started