Where I live we use sunflower oil because sunflowers are growing here while, let's say, olives -- don't. I suppose Italians would use olive oil as a default variant.

Ignore nutritional "guidelines" - look at the nonsensical food pyramid, which even a diabetes doc from 1930 would call bullshit.

What's largely important with oils is the Omega 3,9,6 balance.

There are other considerations after that (GLA/ALA), but it's a good place to start.

So my entire nation, who cook everything with sunflower oil and otherwise eat lots of sunflower oil, are all diabetic and perpetually inflamed?

I mean omega-3, 6, linolenic acid and vitamin E vary wildly between different seeds' oils so I'm sure as an informed consumer one can just leave that flame war alone and figure out by themself that the aforementioned criticism is true for sunflower oils for example but rapeseed oil has a fantastic fatty acid composition and is great for cooking while linseed oil is even better but not really suited for cooking and should instead be limited to cold dishes. To blanket "seed oils" as bad for cooking is just braindead.

To my knowledge, it's specifically sunflower oil that's really high in Omega 6, which is why you shouldn't consume it when you have rheumatism.