All modern Jet Airliners use Oil/Fuel heat exchangers. There are some
smaller Biz-Jets not having them (e.g. Beech Jet, they already had some
problems with flame-outs due to freezing fuel).
The heat exchangers allow to cool the oil while warming up the fuel.
The main problem with freezing fuel is not the fuel freezing in the tanks
but small amounts of water in the fuel freezing either in fuel lines or
in fuel filters. Therefore the heat exchanger is one of the first components
in the fuel system just before the fuel enters the engine / fuel pump.
In the Air Transat Glider incident this heat exchanger delivered the first
hint of the problem: As the fuel flow through the heat exchanger was
a lot higher than normal due to the fuel leak the oil temperature decreased
significantly on the problem engine ...