Anything over 250, and 300 is danger calling Will Robinson. Lower is better but needs to get to least 220 some of the time as that boils any water out of the oil to lower low temperature sludge whopping amounts. The PCV valve then sucks the steam out. The oil type figures in there greatly, true synthetic oil takes a whopping more amount of heat before it damages like old school dino oil does. I would characterize that as more duration rather than ultimate temp as both suffer the higher the temp gets but syn definitely takes it better. Why syn oil can go massively longer periods before changing it.