After buying the V40 a yr ago, I felt to change oil, plugs, filters etc, so used the new oil as synthetic.
Now changing it after 6k...I could tell the viscous character of it compared to the regular[incldg high mileage], oils I ve used in other cars of course...and changed 'em after 4k miles.
So after arguments with my son[who insists no one feel the viscosity, by fingers], I ve put the Castrol high mileage in Volvo to see how that feels, after 4k.
I remember some regular oils even feel like water after having been used for 6k at high temp.

BMW for that reason has service lights yellow and then red.
The engine CPU computes the car's run and oil wear on two factors...TEMP and engine speed[which shears the oil, say tears it apart]. Temp IS a bigger factor.
So the red light to change oil in that case[of BMW] willcome way late IF..
you re running mostly in winter....and at low speeds/engine rpms.

Many other users of synthetics willtell you how less ash and other deposits are left behind in the oil [to blacken it], and other engine cavities....compared to regular oils.

But if some one wants to compare regular oil changed every 2000 miles[I know some], vs changing synthetic after 6000 miles...I will not know how to.