"I read that 15W50 is good up to 220 degrees."
The optimum (best) temperature operating range varies from oil to oil - ie depends on how its been formulated by the chemical engineers.
For best advice on this, check with your hydraulic oil manufacturer.
In my limited experience;
- 93 C (200 F) is a nice, slightly cool, temperature.
- 104 C (220 F) is getting up there, but still well within normal range.
- 100 C (212 F) is desirable,at least once in an operating day, to drive off any water vapour/condensation that may have contaminated the oil.