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?
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.