Specifically, do you think that a 10w-30 in hot south Texas would behave much like a 5w-20 would in a cold climate? Thanks.
You ask a good question because of the Blend of viscosity you want to use.
Remember too, your Fiesta runs a 180~185 degree (F) thermostat - so really - the 10W number is where I would worry.
That is your start up - with properties of a 30W. May be ok for 100 degree Summer days, but not so much for 40F mornings in the Fall.
Most other vehicles run a 190~195 degree (F) - warm up is pretty quick but run in a rather narrow range of heat capacity - sure it's 195(F) but the engine doesn't have much more "rise" after that because of the Latency of the fluids both oil and coolant - that keep the engine stable and in a range for the emissions to be more predictable. So those thermostats make the engine hotter to help with emissions, power and the fuel economy - but pay a price in having to have larger reservoir to exchange heat.
The Fiesta has an ability to operate a wider range of heat - but remember too, has a smaller cooling "jacket" that affects how this heat is passed thru and taken away from the system. The Oil has to do the rest of the job in pulling heat out and away. You want it thinner to help it pull the heat out of the motor - the Coolant can only do so much - the oil has to bear it's burden too. You have two coolers on the engine one for the Coolant and one for the oil. (At Least on mine) This let's them use smaller capacities - cooling on two fronts.
Another aspect to look at is the Fluid capacity of both the Radiator and your Oil by volume - compare this to other vehicles you've owned. You will find many of the 190(F) hotter running cars had greater larger capacities than your Ford, not by much in some cases - but what is in the Jacket of the engine that counts.
You asked about the viscosity - the lower number is your base, 10 - but properties of 30 - so the oil will take longer to pump thru - but pressures will also be different in areas that affect engine timing The fluid will take time to build pressure, and will also need more time to empty from the valve cam..
So the thicker fluid will also take more time to get thru the oil ports - remember you're dealing with a 180 degree F thermostat car, that it's emissions and timing are based upon a given power curve at the temperature - trying to raise up the heat may destroy the ability for the vehicle to take advantage of valve timing to develop that power. Oil starvation can occur because the oil pump is pulsing thicker fluid at a given volume versus the thinner fluid at that same volume.