For what it's worth I phoned Castrol on this subject and the guy on the phone was a 1200GS owner. His view and advice was that the 650/800 motor is best run on mineral oils for the first 5k miles as the engine beds in better that way, then it is fine (and in many respects better) to use semi or full synthetic oils. He added that using semi/full synthetic after 2k miles would be ok but just not ideal. I then phoned a BMW dealer and asked the same question and their view was the same as Castrol BUT "if your bike is still in warranty you are better sticking with the mineral oil since if you have an engine fault you may well run into problems with BMW if not using the oil specified in the manual".
All the best.