A downhole fluid flow control system includes a fluid control module having an upstream side, a downstream side and a main fluid pathway in parallel with a secondary fluid pathway each extending between the upstream and downstream sides. A valve element disposed within the main fluid pathway has open and closed positions. A viscosity discriminator including a viscosity sensitive channel forms at least a portion of the secondary fluid pathway. A differential pressure switch operable to open and close the valve element includes a first pressure signal from the upstream side, a second pressure signal from the downstream side and a third pressure signal from the secondary fluid pathway. The magnitude of the third signal is dependent upon the viscosity of the fluid flowing through the secondary fluid pathway such that the viscosity of the fluid operates the differential pressure switch, thereby controlling fluid flow through the main fluid pathway.