A system for variably resisting flow of a fluid composition can include a flow passage and a set of one or more branch passages which intersect the flow passage, whereby a proportion of the composition diverted from the passage to the set of branch passages varies based on at least one of a) viscosity of the fluid composition, and b) velocity of the fluid composition in the flow passage. Another variable flow resistance system can include a flow path selection device that selects which of multiple flow paths a majority of fluid flows through from the device, based on a ratio of desired fluid to undesired fluid in the composition. Yet another variable flow resistance system can include a flow chamber, with a majority of the composition entering the chamber in a direction which changes based on a ratio of desired fluid to undesired fluid in the composition.