I really don't know if there is a generic replacement, although I would guess there is. I would be surprised if it is an aviation spec hydraulic fluid, most av hydraulic fluids I have dealt with have been quite caustic. I would visit your nearest commercial oil supplier as they will have, or can get, numerous types of hydraulic fluids. Good luck!
Mil-H-5606 is a high Vi mineral based NON corrosive aw hydro fluid with a low vis and low temp PP about -60F VI about 300 or so maybe that is it.
Or exxon univis J-13 or J-26 both used in shock fluids with low PP and high VI also.
bruce