I suppose on a forum where the cars are new, the issue isn't so important as older higher mileage cars. A new engine will be running near to optimum, on any BS approved diesel fuel.
The issue really comes to play when engines start getting full of deposits, inlet tracts start gooing up and EGR valves fill with coke. But as performance and efficiency drop off gradually, it is often not noticed.
So if mpg were to drop gradually, say 0.5mpg per 10k miles, by 80k miles there could be a 4mpg shortfall compared to a new engine. But who's keeping records for engine efficiency against mpg? Hardly anyone really keeps exact records for mpg on a given run anyway, as variables are so wide. Let alone for over 100k, or 200k miles.
But forums are full of "why doesn't my diesel get decent mpg anymore?" "My mpg has fallen off, what is wrong?" type questions, and "I've just cleaned my inlet manifold and EGR valve and the engine runs better, gives more mpg", etc., etc.
Also, unless you were to run two engines and treat them to different fuels over say 100k miles, who will have data on "what is what" for performance, wear, injector efficiency, mpg, deposits, oil contamination, etc., etc., to compare how each fuel works long term.
Another point, who's bothered about engine oil performance over long service intervals? Just a few users, who for various reasons will have shorter periods between oil changes, whatever the CBS suggests. But even that is usually based on viewpoint, rather than the exact science, some have experience, but a gut feeling in most instances.
I suppose it is the old saying "we pays our money..... "