This really isn't that efficient if you think about it. Yeah it would be kinda cool to have a computer submerged in liquid, but if you see what temps those guys are getting w/o a radiator... You can get your system air or water cooled for cheaper. And moving the thing probably isn't easy either, nor is upgrades...
So you are basically doing it for looks which means you should get mineral oil over vegetable oil. Mineral oil is clear and it looks better then yellow if you want to shine LEDs through it.