There is provided a method (1000) of operating a gas turbine engine (10). The gas turbine engine (10) comprises a combustor (16) having a plurality of fuel spray nozzles (403, 404) and a fuel system arranged to provide fuel to the combustor (16). The fuel system comprises: a fuel pump (904); a fuel distributing valve (909) downstream of the fuel pump (904) arranged to distribute fuel to the plurality of fuel spray nozzles (403, 404) and bias fuel flow to the nozzles (403, 404) such that a first subset (403) of the plurality of fuel spray nozzles (403, 404) receives more fuel than a second subset (404) of the plurality of fuel spray nozzles (403, 404); and a fuel-oil heat exchanger (903). The method (1000) comprises providing a fuel to the combustor (16) and transferring heat from oil to the fuel in the fuel-oil heat exchanger (903) before the fuel enters the combustor (16) so as to lower a viscosity of the fuel to 0.58 mm2/s or lower on entry to the combustor (16) at cruise conditions. Also provided is a gas turbine engine (10) for an aircraft.