This invention relates to thickened high-viscosity water based hydraulic fluids. Such fluids are prepared by blending water, organic thickener and conventional hydraulic fluid additives. Organic thickeners tend to lose their thickening effect with time, and particularly when subjected to oxidizing agents, heat, prolonged use in a pump, etc. In accordance with the instant invention, preservation of the thickener, particularly with respect to viscosity loss, is achieved by blending with the final fluid a fluid concentrate, or the thickener itself, a compound selected from the group consisting of ethylene glycol, propylene glycol, propylene glycol polymers and copolymers containing at least 50 percent by weight of oxypropylene groups, a dimercaptothiadiazole, neodecanoic acid, a hindered phenol antioxidant and mixtures thereof.
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