vehicle specific motor oils - why?

[ingenieur]

Might be a simple question but I was just wondering why lube manufacturers offer vehicle specific oils.

To me it seems the only thing that's going to vary significantly from one brand to another is tolerances. So some engines with big gaps would benefit from a thicker oil and tighter engines would be better with a thinner one.

Thickness (or viscosity if you like) does not seem to be the difference though.