I'll add a second vote for Lucas.
I haven't used their products for the particular application you have, but I'm a fan of their thicker version of their PS Stop Leak. As an additive manuf, the key is figuring out the right recipe for seal swellers and conditioners, for a specific appliction. As Ray stated, Lucas stop leak products work as good as any, and IMO, better than most.
1) Track down the proper hydraulic (and gear box, if spec'd different) offering from Lucas.
2) Keep in mind, what it can do is limited by physical damage to seals (cracks, tears), and won't do a lot if your hoses and connectors are in pretty bad mechanical shape.
3) I wouldn't go too crazy upping the oil viscosity - unless you have a heated shop to store it in, or just park it for the Winter.
You may know all of the above already :thumbsup:, but I don't know if you've been wrenching on "stuff" for 1 month, or 10 years....
Rgds, D.