In an aqueous-base drilling fluid for the drilling or servicing of oil or gas wells comprising an aqueous liquid having incorporated therein one or more polysaccharides in an amount to increase the low shear rate viscosity or decrease the fluid loss of the aqueous-base fluid, the improvement which comprises incorporating into the aqueous-base fluid an oligosaccharide mixture composed of arabinose, mannose, galactose, glucose, and xylose sugar units, the oligosaccharide mixture consisting of pentosans and hexosans containing from one to four combined sugar units, wherein the concentration of the oligosaccharide mixture is sufficient to increase the thermal stability or decrease the fluid loss of the fluid, wherein the fluid has a pH from about 7.5 to about 11.5, and wherein the fluid additionally contains magnesium oxide.