Yesterday, I went for a drive through 14 miles of "commuter traffic" that had an average speed of <10 MPH. Sure enough, the temp gauges for both the oil and coolant rose, but were well WNL. The ambient temps were 85 F, relative humidity 28-31%, with a light breeze 8-10 MPH (according to National Weather Svc.). Turned off the road on to the Freeway, and w/i a few minutes the temps settled back to normal for this engine. I was running the car for ten minutes at 65 MPH in 4th gear at ~3500K and there was zero temp increase in either the coolant or oil, during that period.
This weekend, my plans are to fit the 1 Gal surge tank up to the system, on a 13 LBS cap. Then on a day very similar to yesterday, take a test drive on the same road in very similar traffic conditions. My thoughts are that the extra capacity will solve the cooling problem. When it doesn't, then I'll replace the surge tank's cap with a 16 LBS rated cap, then again take the car out on the same road test, in very similar conditions. When none of this solves the cooling problem, I'll have the oil cooler changed to full flow. When that doesn't solve the problem, relocate the oil cooler out of the air stream cooling the radiator.