A drilling fluid formulation is provided, which includes an aqueous base fluid, a synthetic modified phyllosilicate as an anti-sagging additive, an inorganic base, and a weighting agent (e.g. barite). The synthetic modified phyllosilicate contains a clay material (e.g. smectite) and a metal selected from ruthenium, rhodium, palladium, osmium, iridium, and platinum. The synthetic modified phyllosilicate is effective in preventing barite sagging as demonstrated by low sag factor when drilling at elevated temperatures. Rheology properties of the drilling fluid including gel strength, yield point, plastic viscosity, and storage modulus are also specified.