I looked this up recently and will try to recall the key details.
Seastar can be substituted with MIL-PRF-5606H (the "H" is apparently for hydraulic). You should avoid using 5606A as the "A" product is not superclean as is the "H" (and presumably Seastar). I believe the superclean product is necessary to prevent wear.
I had a hard time finding 5606H in a resealable container that would allow me to use just a small amount to top up - you either used the full container or wasted the leftover. I asked someone why I could find "A" in a resealable container but no "H" and they said "if you leave the open container sitting around it won't be super-clean anymore, now will it?" Whatever...
By the way, MIL-H-5606 is the old designation. You want the current MIL-PRF-5606H. The current designation indicates the fluid meets a performance standard of 5606 (thus "PRF").