I didn't have a K100, but I did have a K1100RS for 48,000 in 7 years, and now for the last 19 years a K1200RS
They both ran perfectly on Mobil 1 fully synthetic. I believe 0w-40.
With quality oils I found there was no usage at all (they all smoke a little on start-up) between services, I did try cheaper oil once but found it did use oil between services...
Cheers