A gas turbine engine comprising an oil system and a gearbox, the gearbox is driven by at least one engine shaft, the oil system comprises a pump drivingly connected to the gearbox, an oil tank connected to the pump via a supply pipe and a delivery pipe connecting the pump to the engine for delivery of oil thereto, an anti-siphon pipe is connected between the tank and the delivery pipe to provide an excess oil flow to the engine characterized in that the anti-siphon pipe includes a variable flow valve for regulating the flow of oil passing therethrough such that at low engine speeds an adequate supply of oil is provided to a sight glass in the tank.