First...please read my posts here
Automatic Transmission fluid would work good...it's non-detergent and most times wil be red in color and make for a wicked look, but you'd better be sure you got that case air tight...or your computer room will smell like a jiffy lube. And that's with any oil. Even mineral oil will get a smell to it if its heated up enough.
However, if you read my earlier posts on this...which are on the same page...(not too many posts are made here in the "cooling experiments" section) you will probally agree with me that this just isn't a viable idea. It would be fun to do and one day I will do it, but it will be done with an old rig and mainly used as a conversation piece.