It seems the germans missed their diesel engine properties. Diesel engines can run without any changes on a variety of fuels (the biodiesel talk running around is one of the many cases).
Now, syntetic fuel is manufactored mainly from coal but can also be from Natural Gas (tough in WWII I don't know if they knew this process).
In theory a veichle shouldn't need any alteration to run on synthetic fuel however, the lower octanes in German synthetic fuel however (since it's harder and more expensive to produce higher octane fuel in synthetic fuel (kinda redundant)) could make a conversion needed so the engine wouldn't break or at least see it's service life seriously reduced (If you try to run a modern 95 octane car on 40 octane fuel you won't get such a good result).


EDIT: Here's a good arcticle on Germanys synthetic fuel:
http://www.airpower.maxwell.af.mil/a...aug/becker.htm

Cheers...