Nearly all the hydraulic oil available commercially today has antifoam, antiwear, and detergent additives. That being said I would only add the amount of oil necessary to make it work unless you have thoroughly cleaned and rebuilt the system. Those additives can cause problems in an old contaminated hydraulic system so buy straight hyd oil w/o additives if you can find it. (good luck) If not you may be able to find 5W non-detergent motor oil as an choice. The lighter (re: thinner)n the oil the better. I doubt you intend to use it much anyway.
