Ruger grips are made from a polymer rubber compound called "Santoprene." I did some research on it a few years back for Uncle Sam. It's tough and inert stuff, but like any rubber compound it cleans up better with something that's not petrolieum based which might attack the rubber.
I always use 99% Isopropyl alcohol to remove any lubricating oils, or body oils on my SP101; it's cheap and effective. 70% rubbing alcohol is too much water IMHO. Just dry it well with a hair drier or paper towels before putting it back on.
Hope this helps.
John