I assume it is a Seastar system. You can't go wrong using the Seastar brand of fluid. There are other options that are less expensive. So if I am trouble shooting a leak or flushing out my steering, I might use the less expensice hydraulic fluid since most of it gets tossed. For general topping off and minor bleeding, I use the genuine Seastar brand.
Use the Seastar especially if you are mixing. They also sell a screw in fitting for the top of the steering unit, clear plastic tubing and cap end that fits the bottle so you don't make a mess. Can get off Amazon.