The principal requirements of a premium hydraulic fluid are proper viscosity, high viscosity index, anti-wear protection (if needed), good oxidation stability, adequate pour point, good demulsibility, rust inhibition, resistance to foaming, and compatibility with seal materials.

John Deere 5105 Specifications

Viscosity is a measure of a hydraulic fluid’s resistance to flow. It is a hydraulic fluid’s most important characteristic and has a significant impact on the operation of the system. When a hydraulic oil is too thin (low viscosity), it does not seal sufficiently.

Fluids with low viscosity have a low resistance and shear easily and the molecules flow quickly; high viscosity fluids move sluggishly and resist deformation. Some liquids, like pitch, glass and peanut butter, have such high viscosity they behave like solids.

The higher the viscosity of a liquid, the thicker it is and the greater the resistance to flow. Temperature will affect the viscosity of most materials….Viscosity Scale.

So glycerol is the most viscous.

One of the reasons it took so long to identify tar pitch as the slowest-moving liquid on the planet is because it looks like a solid at room temperature.

Ethyl alcohol is the least viscous fluid that you could (A) acquire a large enough quantity to swim in, and (B) not die from dipping your body into it.

A non-dense, non-viscous liquid like water will appear “thinner” than a dense, viscous liquid like molasses. The viscosity of water is 8.90 × 10−4 Pa·s at 25C, the viscosity of acetone is 3.06×10−4 at 25C. In my interpretation of your wording, this would make it seem like a thinner liquid than water.

Well, I cannot speak for all kinds of alcohols, but the common ones (methanol, ethanol, and isopropyl alcohol) are a little less dense than water. All three of these have densities of about 0.79 g/cc at standard temperature and pressure, compared to 1.0 g/cc for water.