Mineral oils are formulated to run at 250F; more than that and they'll need to be changed at the OEM recommended interval - don't go longer. Synthetics can run hotter without "damage" to the oil, but you'll be stressing the parts- seals will get cooked and harden (then leak) and parts will come closer to contact (hotter oil is thinner oil) so running hotter isn't better just because you have a synthetic. In many cases, targeting a temperature of 230F is good, 240F max, since then it's hot enough to boil off any water that may collect, well within the oil's comfort range, but not hot enough to stress the lubricant or any parts.