An invert emulsion drilling fluid, and a method of drilling with such fluid, having improved rheology at low mud weights and high temperatures. The improved rheology is effected with addition of a rheology additive of the invention comprising fatty dimer diamines or dimer diamines and an organic acid or ester of the acid. A nonlimiting example of such a rheology additive comprises a C36 fatty dimer diamine and adipic acid or dimethyl adipate.