Abstract: The present invention relates to a fluid comprising a compatible blend of (A) a polydimethylsiloxane and (B) a polybutylene wherein the viscosities of these two components are substantially identical such that the viscosity of the blend is less than the viscosity of either component (A) or (B), each viscosity being measured at the same operating temperature and pressure. This synergistic viscosity reduction is enhanced when particular moieties are present in the polydimethylsiloxane and/or polybutylene components and as the polydispersity of the polydimethylsiloxane is increased.