Cool cool. I am only sharing my experience and observations. I generally watch my oil and coolant temps like a hawk regardless of where I drive so it was very easy to notice something like this.
Turning on the A/C, especially in traffic, may temporarily bring down temperatures as the auxiliary fans are always on when the A/C is on to move air past the condenser and hence also move air past the radiator. However, this will only be temporary and you'd notice the difference at idle with no load on the engine.
I respectfully disagree. When I noticed the unusually lower oil and coolant temps it was a day when I spent just over 4 hours of continuous driving which included in-down-town traffic, highway driving with a few series of WOT pulls and some generally slower city driving (you know, inner city back streets, stop sign to stop sign). Not once for that period of time did the oil and coolant temps rise above 190F.
I guess what I am trying to say is that I believe
that managing coolant temperatures is far more important than managing the oil temperatures. It seems that if we have sufficient radiator cooling capacity, the entire engine block will be cooler overall. This in turn will help manage the oil temps naturally.
Another reason why I believe that the coolant temps govern the oil temps is because there is FAR more coolant by volume than there is oil. Also, the radiator is far bigger than the oil cooler. You'd expect to see the coolant temps to variate more than the oil temps which seems to be the case. Also, the entire engine block is water jacketed, so if you have cooler engine block, the oil won't get heated up as much when it is draining.
That's my theory and experience. My car is currently being serviced and I can't retest it, but I am fairly confident that my tests would be easily replicated.