As above really, what exactly about the tyres makes them "MO"? Is there any good reason I should not pick up a set of bargain "MO" tyres and fit them to my z4?
They might have stiffer sidewalls, or composition and be tweaked for a car that is 20-50% heavier than a Z4 - or there may be no difference and be purely a marketing/money-making exercise between the tyre manufacturer and Mercedes!
MO tyre have a bit different "seat" pattern for Mercedes rims. But you can run them on Zed's rims without any issues.
edit: not "seat", bead would be the correct word