I posted a response to the "ROW Oil Recomendations" discussion thread, but I think it's worthwhile to cut and paste that response in here, to address your concern.
If your cooling system is working properly, the internal engine temperature will be hardly any different at full operating temperature regardless of whether it's the deep of winter or the peak of summer.
If you have a Scangauge, you'll be able to see your engine coolant temperature - this will reflect your internal engine temperature, which is what the oil is subjected to.
For me at least, in Australia, right in the middle of winter, my engine coolant temperature is 78 deg C in daily driving, and in the peak of summer, when I am towing my caravan, it is around 95 deg C max. Australia's summer heat wave temperatures can hit around 40 deg C, which is 100 deg F.
So my observation of the coolant temperatures is a maximum variation is around 17 deg C at very opposite driving conditions.
In my opinion, this sort of temperature range is hardly noticeable by the engine oil. So I don't think it's worth the trouble of switching from a "light" oil to a "heavy" oil during the seasonal changes.
The engine will suffer from poor fuel consumption when using the heavy oils and the benefits will be highly dubious. The heavy engine oils may be useful when the engine has reached very high mileage and very worn - plenty of time much later on before needing to think about using a heavier oil then.
That's my opinion anyway!
Buy a Scangauge or some other OBDII monitoring device and see it for yourself - if the cooling system is working properly, the engine's temperature will be very consistent.
It would be far far more important to regularly change your automatic transmission fluid, as the cooling system for the transmission is no where near as good as for the engine. I see that the transmission fluid temperature fluctuates over a very very wide range.
I would say to you that you can just stick with 5w-30 fully synthetic engine oil and just change it regularly, maybe every 10,000 kms would be fine.
But definitely get your transmission fluid serviced regularly, even though Toyota specify that it's a "lifetime fill" gearbox. I would suggest that if you drive in sand a lot, you may need to do a transmission service every 60,000 to 80,000 kms or even more frequently if the fluid gets burnt from the constant high loads of driving in the desert.