The best oil for my fiesta ST 2014
If you're only changing the "winter" weight, it doesn't do anything at the higher summer temperatures, as evidenced by the chart in your link. Unless you are picking it because the additives in the 0w or 5w are different, the oil behaves like a -20 weight oil regardless of what the "winter" weight is. If you want to change the viscosity during the summer months, go up to -30 weight. I've done this and the engine feels smoother but Ford doesn't recommend (see my below comment).I've been using 5w-20 during summer and 0w-20 on winter. I just use any conventional oil since I change it every six months regardless of mileage. My 2014 Fiesta is still running smooth just like I first got it brand new. This is my resource Best 0W-20 And 5W-20 Oils For 2021 | MCrunch.
To put it simply, companies buy the same base stock from refineries and add their own additives so when you buy one brand instead of another, you're buying the same base stock, just a different additive package. Depending on where you live, what's in the Castrol bottle might be the same base stock as the Pennzoil.The best oil for my fiesta ST 2014