From my view some views expressed in the "Old School" maintenance are just that - old school thinking. Really the new synthetic differential and manual trans oils are not breaking down at 30k miles. A better investment is to replace your Fill & Drain plugs as applicable with the Dimple magnetic plugs. Stock BMW plugs are not magnetic. Gears, bearings, generate fine particles - getting them all out with a mere fluid change is not feasible.
http://www.drainplugmagnets.com/magn...plugshome.html