A treatment fluid comprising an aqueous liquid and an associative polymer additive. The associative polymer additive increases the plastic viscosity (PV) of an aqueous liquid by more than at least 50% and wherein the associative polymer additive increases the yield point (YP) by no more than about 30% of the corresponding increase in the PV relative to an aqueous liquid without the associative polymer additive. Moreover, the viscosity of the aqueous liquid comprising the associative polymer additive is capable of maintaining a viscosity of greater than about 20 cP for at least 20 minutes at temperatures higher than about 275° F. In some cases the associative polymer additive may be a hydrophobic alkoxylated aminoplast.