I have an UTB 445 which is I think basically the same engine. My power steering reservoir also empties over time - say 6 months. The hydraulic pump seal leaks into the timing case 7 hence the sump.
My manual however, says to use engine oil in the power steering (non hydrostatic type) & transmission. I use Universal Tractor Oil which is 15W40 for everything (inc the oil bath aircleaner) except the final drives & front diff. This has been fine for the 8 years I've owned it. Twice I think I was concerned about the sump level being too high, so I drew off a pint or two. Normally with medium use, it burns about what the power steering leaks, so all is fine!!