If I remember, the Fsi was designed to run very lean on light throttle with a direct injection system and there were rough running problems encountered and I think there were cases of engines damaged by too lean a mixture. Also because the petrol is directly injected into the cylinder, the EGR gases containing combustion products left sooty deposits in the inlet tract and on the back of the inlet valves, which built up as there was no petrol in the mix to act as a solvent. I'm not sure how/when all this was resolved.

what does 'lean mixture' mean exactly?

It means that there's excess oxygen in the cylinder when you drive gently (like a diesel). It didn't seem to work fantastically in practice and I think vW quiuetly dropped this feature.

I wouldn't worry about the type of fuel that's used. Most drivers are sensitive to fuel prices and buy the cheapest available. Most people buy from supermarkets. Most people don't have problems with their fuel systems.

There was another genuine problem back in the early 90s when standard fuel often contained a lot of sulphur. This was corrosive to Nikasil cylinder bores and quite a few engines had to be replaced because of it.

But this has long since been sorted.