In the air, TAT is more relevant than OAT. OAT is interesting, but TAT drives fuel temp. Freeze point for Jet A1 is about -47C. Get close to that, and you can start getting fuel gelling in the tanks, especially wing tanks. Most aircraft have a fuel temp warning at about -42 C. You can raise that TAT for a given OAT by going faster. For flying over the pole, where OAT was often -65 C, United would sample the fuel uploaded on that flight to determine the actual freeze point. It was often a couple degrees below spec (that's a good variation), then datalink that to the crew so they could better manage fuel temp. Incidentally, when the fuel gets to the engine, it is warmed by the oil cooler, so you're only pumping super gold fuel into the engine itself.