sorry I have been terribly silly, I made a mistake in maling 10*2 - 2 ....

Do you mean that the same argument for $X_{10}$ works for $X_{18}$?

Does this help also for $X_{18}$? It seems just set a correspondence between certain types of Fano 3folds.

@Sasha: sure it was a typo! $(d+2)/2$, thanks. But still the question stays open.

Still that degree 8 (and sectional genus 5) K3 must have something to do with the cubic fourfold. In the $\mathcal{C}_d$ cases where there is an associated K3, the surface has degree $d$ and genus $2d-2$. This is why I suspect that when such a cubic 4fold has an associated K3 (for example if it is in $\mathcal{C}_8\cap \mathcal{C}_{14}$ or 26), then this K3 may be related to the octic.

@RP: good point, I nedd to think about it but probably you are right. But still the question makes (some) sense - I guess.

@Donu : yes, of course that’s what I meant