UPDATE: I looked at a couple of auto parts stores, and Tractor Supply, and found hydraulic fluid that didn't have sufficient details on the label to know if it would meet the specs. So I passed on those, went to West Marine, and paid $27 for a quart of the SeaStar fluid that says "Hynautic" right on the label. (I later found it online for about $18 / quart, but don't know if that included shipping.) Next time - if there IS a next time (the fluid really shouldn't ever go away), I will investigate some aviation hydraulic fluid that meets MIL spec 5606, and see if I can fine some for less $$, but I was in kind of a hurry this time - got a Bahamas trip coming up fast!
Oh - I'm pretty sure the PO used Dexron ATF in this unit - what's in there is red, and the SeaStar fluid is almost clear. It seems to have worked just fine for a lot of years.