I don't know of anything that you can add to hyd. oil to stop it from leaking. The best advice i can give is to either replace the shaft seal, or seal the threads on any fittings that might be leaking. I dont how old your mower is, but over time the shaft seal can wear. That seal can also wear a groove around the shaft where the lip rides. In that case installing a shaft seal won't work. You'll need to install a new shaft in the hyd. motor, or have the old one welded up and turned down then polished.
You can also just put a rag under it if it's not that bad.