As I understand it, without a pump, the 1500 uses natural convection (induced by heat) in the coolant to circulate. If you use more than 30-40% antifreeze, the mixture is too thick and instead of the usual raise in boiling point one would expect with antifreeze, you get a lack of convection/circulation and failure to cool the engine.
So a thinner mixture is preferable for better convection. Some have suggested water with a wetting agent--of course then the fluid boils at 212 degrees (or slightly higher if adjusted for pressure).
I imagine anything less than 30% antifreeze will work, just don't let the engine freeze in the winter.