You can find hydraulic oil with a spec very close to seastar oil at the next citroen car dealer , the reference is = LMH OIL
Comma L.H.M. PLUS 1L
Avoid DEXTRON 2 or dextron 3 in seastar oil pump as the viscosity is a bit higher, and the inside checkvalves don't react fast enough with this oil, you feel to have a heavy steering system with DEXTRON on seastar pump.
for the lift or tab pump using oildyne pump, I use hydraulic oil 46 viscosity for years.
for the trim pump, per Mercury informations:
Quicksilver power trim and steering fluid #92-90100A12. If unavailable use 10W-30 or 10W-40 motor oil
be careful, better to use oil with anti foam additives
other possibilities for sure from other members.
Hope this helps!