I don't know about optimal...below freezing would be optimal
...but anything under 120F should be great. You can probably even go a bit higher without damaging it in the long run. I have been running my K6-3 at like 130F for over a year without a problem. The junctions will break down completely at 150F junction temp, but lifetime will go down at less than that.
"Water-Cooled CPU Runner"