What application is this and what type components? It would be beneficial if you could reduce the temp to 50C or so regardless of what type oil you run. Maytag
Be aware that for those pressures, any foam or air in the system may cause diesel effect and may brake lines. Use a high quality hydraulic, if possible with dispersant characteristics, Europeans call this HLPD according DIN norm.
Perhaps a synthetic engine oil 10W-20 with no viscosity improvers will do the trick.
My experience with high pressure and temp was in Plastic Moulding industry, the Injection moulding machines. Good oil recommendation would be 1. KV 40 - 46 to 68 2. VI - 140 3. Super clean Hydraulic Oil
There have been complaints of Hydraulic becoming oxidised (black) with in a few hours. Probably a result of "dieseling."