As a general rule of thumb - anything over 220deg F for dyno oil and 250 deg F for synethic are my cut-off points. It depends on the oil, the amount of anti-oxidants, and the time it's at that temperature. So, lets say I touch 220 on a dyno oil... probably no big deal if it's a high quality oil and it's only instantaneously. If I'm sustaining 210 for more than a minute, I'm definitely going to put in a cooler.
I reccomend a cooler on the 4ag because they're prone to rod knock. Furthermore, the blacktop has lightened internals and the conrods are notoriously weak. Without a cooler, 3 laps at 110deg F ambient was enough to run the oil temp up to 250 deg F on my 4age.