Could anyone with an unmodified 6G74 & with an oil temp gauge answer this?

How much higher does the oil temp get driving fairly quietly, in free-flowing traffic, on a hot summer day (say 35 plus) compared with mild weather?

What other factors cause an oil temp increase - towing/heavy traffic/spirited driving?

I'm assuming the cooling system is working efficiently

I cannot answer as a person with an oil temp gauge, but I would guess that the oil temperature will be approximately the same as the coolant temperature as both oil and coolant cool the engine. The thermostat will maintain engine coolant at a certain temperature summer and winter and the oil temperature will follow to a certain degree. Oil is continually flowing through the engine the same as coolant. Variables would include loading the engine which would heat the oil due to friction etc, but the coolant would be subject to these same influences. I've had Jaguars in the past - they had oil coolers fitted plus ribbed alloy sumps but they boiled coolant usually before the oil would overheat, but then Jaguars were designed to impress.
I think the main thing to consider is that engine oil performs the same job as coolant - both cool an engine.

I've heard that oil temps would be higher than your coolant temps. But I haven't confirmed this either.

One thing I have done though, on a motorcycle is run a water temp sensor and gauge in the engine oil. Under all manner of conditions that gauge/sensor didn't read any higher than it did when run in their intended conditions. Telling me oil and coolant temps don't vary so much. But there has to be a point where the oil temp will go higher.

Suzuki had a few air/oil cooled bikes. My gsxr was air/oil cooled. Massive oil cooler on it.

In normal conditions, like you've suggested, I don't think oil temps would be an issue. The engineers at Mitsubishi specified an oil standard for the job.

Oil will generally be around the same temperature as coolant, until you start doing some spirited driving. I can only give you an example of when I had my Evo, as it had an oil temp guage. Keeping in mind it was heavily modified, and came with a very good engine oil cooling system from factory; Standard oil temp in every day conditions was around 90 degrees C. A spirited run up some mountains, or a couple drag races would see that number get close to 110 degrees C. Nothing to worry about (FPV's/HSV's are well known for oil temps hitting 150+ during track testing for our local automotive magazines) The coulant, I couldn't tell you a comparative figure temperature wise. The temp guage never moved on the dash, but that doesn't mean a real lot, as those guages are generally not very accurate. At a guess I would say the coulant got to around 100 degrees C when the oil was 110 degrees C.

To answer the OP question: What other factors cause oil temps to increase? Anything that causes excessive engine friction - Hard revving, high loads, towing, long inclines etc. Assuming your cooling system is working efficiently, I wouldn't imagine the difference in oil temp would be that great between a day that was 25 degrees C, and another day that was 35 degrees C. I certainly didn't notice much difference in my Evo - maybe a degree or two.