Over on the Mercedes forums there are some folks either working for Mercedes or with really good connections. They said that M1 0w40 was found by Mercedes to start showing it needed to be changes at the 13000 mile mark. This is of course a generalization but I presume it must be a conservative limit to account for varying conditions. It is interesting that Mercedes stopped FSS and fix maintenance at 1 year / 10k now. The FSS systems were apparently quite sophisticated with oil condition sensors.