Usually Propylene Glycol or Glycerol is used up to ~40% in (usually sesame or cottonseed) oil based preparations, I believe that these are the "polyhydric alcohols" (an erroneous term) you refer to. I don't know about carbonyl solvents either, but usually 5-10% Benzyl or Ethyl Alcohol, ~1% Chlorobutanol, or even <1% phenol are used as preservatives, to prevent bacterial growth.