While a lot of fun to do the whole oil submerge thing (done it in a "designer" fish tank for my media center), unless you use very expensive industry grade non-conductive coolant (rather than just mineral oil), it's actually got worse cooling properties than water and also it's usually got a viscosity many times higher than water so you need a really heavy duty pump, so it be noisy as hell because of it.

So I can't recommend it for any big power guzzling PC as cooling that oil is certainly not as easy as water.