I use the stuff from an auto shop, Anti Freeze Treatment, seems to prevent slime in the water, been running for 2 years straight, no slime yet. Have not flushed the system with this stuff.
It's not the coolant you would have to worry about when using a restrictive water block - it's the tubing ID size and the pump that would have the most influence.
I've got the DD D-5 pump and using 1/2" ID tubing so I think that part should be okay.
I just didn't know if the viscosity of it would matter. I'm guessing since it's non-conductive there aren't any little particles in it to get clogged in the jets.