I agree acetone drys wood, but not excessively when used rarely. I don't agree any oil or similar material should be added to wood to help preserve it. I have tried to support my views with empirical evidence including using a microscope to inspect the grips before/after cleaning. More data in these threads:
http://forum.lugerforum.com/showthre...hlight=acetone
http://luger.gunboards.com/showthrea...hlight=acetone
The good news is, the hardwoods used in Luger grips are tough and can withstand a great deal of abuse - oiling, improper cleaning, too much/too little cleaning etc. In most cases most of the time, if the grips are not clearly in need of cleaning, they should be left alone to match the appearance of the pistol. In my opinion grips that look like those in post #1 are fine as-is.