I'd agree that the complexity of real-life conditions makes measuring oil-related fuel economy improvements tricky. Instead, the industry uses lab based, fired engine tests to measure fuel economy & fuel economy retention. Under such controlled conditions, you can compare oils reasonably accurately. I'd agree with you that oils of the same viscosity grade & spec tend to give you identical fuel economy. Shell or Mobil will be no different to Tesco own-brand. Dont be suckered in by fancy talk of friction modification. Other than Moly at very high (and eye-wateringly expensive!) concentrations, I never found an FM that worked consistently. However low viscosity oils, all things being equal, DO work. I use 0W20 in my car. I once did a long motorway trip & measured 76.8 mpg. For a simple, non-hybrid, petrol powered car, that is quite exceptional! Just so that it's clear, in the UK, you don't need the '0W' in the same way Canada or Finland does because it never gets that cold here. However that 0W rating means the oil is relatively thin when you start-up & throughout the warm up phase. That's where you'll accrue a good chunk of any fuel economy credit. In the US, 0W16 is already displacing 0W20 & 0W8 is fast coming down the track. I know some folks are highly resistant to the change but thin oils are here to stay.
Thanks for your expert advice
dont matter what you put in that, the engine will blow up sooner rather than later.
Unless you got a brand new car with a fancy engine this stuff is good enough for most modern cars.
Independent scientific testing disagrees with you. 10 minutes with Google will turn up many many results but you sound like you've made your mind up already so it's not worth the effort. I'm not sure if you are ignorant or just stubborn and like conspiracy theories.