I was just thinking, wouldn't a thicker fluid like maybe some good old 10w30 be able to absorb more heat and thus not need to circulate as fast as your typical water-based setup? Or would it not absorb heat quickly enough?

it'd be more stable at higher temperatures, but you'd be having a few more issues than your fluid breaking down if you did hit those temps. that and i doubt it would heat as efficiently. i would actually guarantee that much. then, pumping it would also be more of a chore because of its viscosity. more work for the pump, probably a noisier pump.

im not actually sure that there is a more efficient fluid for heat transfer in the temperature ranges that you'd be dealing with than water... not that it doesnt have its downsides, but in a properly maintained loop i would think it'd be the best option.

There are some PCs which operate in tanks filled with oil that serves as a coolant.

While this is probably not quite the way of doing it that you're thinking of, here's some interesting info re: oil as a coolant:

http://www.pugetsystems.com/submerged.php

You can buy a kit to do this http://www.pugetsystems.com/aquarium_computer/V3/both_module.php

Or I can do it myself for $600 less haha.

Quite the reverse, I think - oil has only half the heat capacity of water.
According to wikipedia water has the second highest specific (by weight) heat capacity of all known substances. Ammonia appears to be the only contender, and is ~50% better at high temperatures (100 C), which you hopefully won't see from your CPU.

My physics is rather rusty, but I think this means that e.g. oil would need to circulate faster to remove the same amount of heat. And getting an efficient (turbulent) flow in the water blocks will be harder the ticker the fluid is.

There are always disadvantages for coolant fluids other than modified water.

Fluorocarbon always come to mind (Liquid cooled super computers) - Costly stuff.


Or a Liquid Metal...

http://en.wikipedia.org/wiki/Galinstan

However it corrodes various other metals (like aluminum...) makes it very problematic.
(Also costly...)