A subterranean location accessed via a wellbore is treated with an oilfield fluid containing a viscosifying agent which is a polymer and a cross-linking agent which can undergo the Diels-Alder reaction to form a gel. Conditions at the subterranean location alter fluid viscosity by bringing about the Diels-Alder reaction to increase viscosity or the retro Diels-Alder reaction to decrease the viscosity of the gel.