If you have a general industrial requirement, ZDDP chemistries have been used for decades with success. They are inexpensive and provide good antioxidant properties in addition to antiwear properties. This would be an appropriate additive for leaky hydraulic units. ZDDP has a lot of challenges however in more demanding applications and is viewed to be less environmentally friendly.
If your unit is operating at higher temperatures, is at risk to water exposure and/or has a low make-up rate, I would suggest a zinc free additive system. There are many zinc-free antiwear additive packages available depending upon your application.