Hsmo fluid

Have begun to get power hydraulic noise from (pump). It sounds as if HSMO
fluid may be low. Would like to know how one goes about filling resevoir.
Jag manual describes draining via disconnected return lines, but does not
indicate how to fill up with new HSMO. Does resevoir caps untread etc. Also,
is Hydraulic system mineral oil readily available or something I must get from
dealer or parts supplier. Any help would be apppreciated. Symptoms are loud
ticking which increases with Revs (pump rotation). It goes away when pedal
pumped quickly, but will return upon constant pressure. Pedal is fine.
Braking is fine. No low pressure warnings, etc. My thought is perhaps HSMO is
low and air is entering pump. Car has been converted from self leveling. Any
thoughts would be appreciated. Pete