Okay, I'll admit, the post title came out a little more on the clickbait side than I would like bu it was either that or a wall of text for a question.
Now, to the issue that brings me here.
I've recently bought a car and had a service/maintenance done on it, just to know what could be hiding in waiting to trouble me.
I usually buy all parts necessary and hand those to the shop and this time was no exception. The novelty was that for the first time ever, the mechanic told me I should be buying a specific brand of engine oil for the car in question.
The car I bought is an Opel Astra, with a GM diesel engine, turbo assisted. The engine takes 10W40 and I usually buy ELF oils for my cars. This mechanic told me ELF is best suited for Renault engines, while Opel (and all german made engines at large) uses Castrol.
On paper, both manufacturers make their engine oils to the same industry specs, so why this splitting of hairs?
Project Farm on YouTube has a range of videos on motor oil. Maybe one of those videos can help you.
https://www.youtube.com/playlist?list=PLjT3B9r2z3fXlvbP3tb9e056Q8O5krrCW