I don't think my oil temperature has exceeded 290 degrees Fahrenheit since I installed my Spud Oil Cooler. My XR650L will only approach that oil temperature when I am ascending very long, steep mountain trails, or riding WOT down the freeway for 35 miles, or more. However, it's not hard to exceed 275 degrees while climbing 2,500-4,000 vertical feet in the Rocky Mountains.
If you can keep oil temperatures below 275 degrees, that's great.
However, I think 300 degrees is a very practical, maximum temperature which will still prevent heat degradation of the engine oil.
If you remove the stock air scoops, and ride the bike hard, an oil cooler is a very wise investment.
The oil cooler not only reduces oil temperatures while riding; it also cools the oil much more rapidly after you stop the engine. My oil cooler not only radiates heat from the cooler itself and its external oil lines; as the external oil cools it also reduces the oil temperature in the reservoir to which it is connected.
At any given time, a large amount of the engine oil resides in the oil reservoir and downtube. Therefore, an oil cooler will greatly reduce operating oil temperatures after you restart the bike, and the much cooler, external oil is returned to the engine.
If anyone is interested, here's a link to the installation thread for my Spud Oil Cooler, which costs less than $100 to fabricate.