Just a quick point reading this.

I do not work for an oil manufacturer, I am a seller of 5 different brands.

I can only point out the facts and it's up to you which brand/grade you choose.

If you were to be offered 10w-40 by Castrol, Silkolene, Fuchs, Mobil or Total which one would you choose and why?

Personally, based on technical data, I would choose the one with the best spec (highest viscosity index) as it's a better oil.

It may be surprising to learn the the one with the highest VI index is Fuchs! Who are they I hear you cry? Well they're only the largest independent oil blender in the world with sales of 1,000,000,000 euros per annum and factory fill positions with VAG, Mercedes, Ford, BMW etc. They are also the parent company of Silkolene here in the UK. (they also make top synthetic oils)

You learn something every day and that's where we can help.

Cheers
Simon