BMW OEM starters are all I know. But I can't see why yours wouldn't all be very similar. They are a simple armature suspended between 2 ball bearings. There is some drag created by 2 heavy duty brushes and residual magnetism in the field windings. (Some non-OEM starters may use double-sealed bearings so that would be extra drag.) Bottom line... they don't spin as easy as (say for instance) a common electric motor. If you give a starter armature a big spin, they won't turn more than 1 to 1.5 revolutions due to the added drag. But on the other hand, if it's held in a vice you should be able to move the armature using your 2 "baby fingers".... if you had your Wheaties this morning. 

In OEM starters it's the grease in the bearings that oxidizes and becomes the consistency of Play-Dough which spoils everything. That takes 10 or more years. The added drag makes the starter pull more current... which is harder on the battery and all the cabling and connections. So the battery runs down more quickly.

If you don't have the physical drag, slow starter can also be caused by the 2 cables and 4 high current connections. Just a tiny bit of corrosion at any of the 4 connections on the 2 big cables can wreak havoc and drastically reduce the power that should be available to the starter. Thus, obviously: Reduced electrical power IN = Reduced starter power OUT. 

Both OEM battery cables are also highly suspect, because they are known to be poorly made, and susceptible to the older battery types that sloshed acid all over the 2 terminals. The Positive cable is especially prone. Please also see THIS THREAD if you haven't already.

Hope this gives you some ideas.

Thanks Richard, but the valeo and denso starters have a reduction gear and a one way clutch on which the pinion gear is mounted. 

The free spin direction of that clutch is what is in question on my starter.