William, you are a star! Thank you so much for this!
I agree with you on the washer...even the prints on it don't go with the assembly.
The diameters are roughly the same here (quick look with the ruler). Seeing now how it lines up in there, I can understand the logic. It allows oil to pass through but not back...just like the awesome members on this forum guessed. Now, considering that oil still goes back through another valve I see no interruption in the flow nor any "starvation" of oil when engine is off. I'm I correct in saying that? Nevertheless, the car seems to run just as good without it but I'll make sure to reinstall it when the day comes...finalising on the solution
Cheers buddy
BMW X5 4.8iS 2005