All fluids are viscous. Some fluids have low viscosity, while other fluids have high viscosity. I'm not aware of a common word, even a common technical word, that will fit in your blank. "Inviscid," from another answer, is not in common use and conjures a technical meaning that I don't think you're after.
A similar example from the sciences: all objects contain heat. The analogy is that "viscosity" is to "viscous" as "heat" is to "hot"; you're searching for "cold," but I don't know that you'll find it.
I would abandon the parallelism and describe what the fluids do: treacle is viscous, but alcohol flows freely.