The factory recommendation is gear lube in the rearend. I use 80w90. Several here use a high quality hydraulic fluid like hytran instead of the gear lube. One could affectively argue the use of either. Hytran allows you to use the same oil in both the hydraulic and rearend reservoirs. In addition to being the factory recommendation, gear lube might be less prone to leak past the axel seals and other areas.
I use hytran in the hydrostatic power steering system. Automatic transmission fluid might be helpful in cold climates in the earlier power steering systems. The hydrostatic steering used on s.n. 8262800 (1965) and later 430/530s and the 470/570s should function well by design with hytran in all climates.
You can still get the 3pt piston seal and wiper from case to fix the leaking of oil from the hydraulic reservoir to the rear end. - Joe