Why does propane gas have the lowest viscosity out of all other factions?

Viscosity is a property of liquids that is a measure of how sticky they are so can be measured by the speed that they flow. For example, honey is more viscous than water. The longer the hydrocarbon chain the more viscous the fraction. So methane and ethane would be less viscous than propane - however, remember this is strictly a property of liquids and all 3 alkanes of these are gases, so we would have to cool them down considerably to measure the viscosity at all.