If you own the car then it's worth looking after. Good petrol can only help.
Is it worth it? I don't know but at least if I'm paying too much I feel good about it.
The car feels smoother, mpg is seems better and more responsive.
Opinion over 4 years only.
No conclusive evidence either way that I can find.
Oil.
40k and not used a drop in between services. 3-4 litres a year? On a petrol engine?
I'd be worried.