Anti-freeze (ethylene glycol) is actually about 18x thicker than water at or around 20-30C. Anti-freeze does two major things - it lowers the freeze point of water for people who live in snow-bound countries, it raises the boiling point of water which under pressure allows cars engines to run at optimal temperatures.
Anti-freeze has a specific thermal capacity about 1/3rd that of water, meaning that it heats up more per unit of energy, and its added viscosity increases the effort required to reach turbulent flow conditions, effectively lowering the Reynold's number for any given flow rate resulting in lower thermal transfer. Glycol becomes substantially more "runny" at the ~120C operating temperature range for car engines, and this coupled with increasing the boiling point of water is why it "helps to cool a car engine more efficiently".
It only "improves" the thermal properties of water in its alteration of water's freezing and boiling points, thereby extending the useful operational temperature range of water. It does not actually make water a better conductor of heat.
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