A method of operating a gas turbine engine, the engine including an engine core with a turbine, a compressor, a combustor arranged to combust a fuel, and a core shaft connecting the turbine to the compressor; a fan upstream of the engine core; a fan shaft; a main gearbox that receives an input from the core shaft and outputs drive to the fan via the fan shaft; a primary oil loop system to supply oil to lubricate the main gearbox; and a heat exchange system to transfer heat between the oil and the fuel, the oil having an average temperature of at least 180° C. on entry to the heat exchange system at cruise conditions. The method includes transferring heat from the oil to the fuel to lower the fuel viscosity to a value of less than or equal to 0.58 mm2/s on entry to the combustor at cruise conditions.