If it will stay in the shock, any hydraulic fluid will work. Motorcycle shock oil for instance.
One of my front shocks on my '50 started leaking right after the rebuild warranty. Hydraulic fluid ran out almost as fast as I poured it in. What the heck, may have to come off and be resealed. Nothing ventured, nothing gained. Filled that rascal with 85-140 gear oil. Thousand mile or so and still full and works.