The 4G37 needs 4 litres of oil.
The 4G15 needs 3 or 3,5 litres i'm not sure at this moment.
But those are 2 completely different engines. The 4G15 is an Orion sereis engine and the 4G37 is a Saturn series engine.
I don't think the oil cooler holds 1 litre of oil.
I honestly can not tell. Because I don't have material to compare it to.
I can imagine there is a bit more resistance in the circuit because you make it longer and twistier because you feed it through an extra radiator.
What are you planning to do, that you are considering to add an oil cooler?