I was certain that a silicone O-ring was a superior O-ring material to use for my corny keg aging vessels. However, a google search revealed several O-ring compatablility charts all which show that both natural rubber and Buna-N type O-rings are more compatible with ethanol (Ethyl Alcohol) than silicone.
Silicone is rated as a "Fair" O-ring material for ethanol, usually OK for static seals in the first chart. The second compatibility chart shows it as being good.