As I recall from a previous discussion, the Seastar fluid is an aviation grade hydraulic fluid. There's some Mil Spec number attached to it, but I don't recall what it is off-hand. I believe that any light hydraulic oil will be fine (heavier stuff will work, but will increase steering effort.) Perhaps a stop by your local aviation supply place might be in order as well (assuming you can't find the Seastar branded stuff.)
jky
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