Hello Christian:
There is disagreement about cleaning grips and how this impacts value. Personally I clean mine as I see it as similar to cleaning the gun. I don't think you want to use water as you could get swelling or possibly warping. See Buchanan's discussion of grip cleaning methods. The link is
http://www.lugerforum/gripclean.html Also, many folk do not recomment linseed oil, although Brownells sells a specialized stain, Plinkington Linseed Oil - Red/Brown that works well. It gives the a rich look. If you want a very good job of cleaning the grips, sent them to Hugh. He does amazing work....