Should not something for the Griso/Stelvio work?
Curious, once the oil gets to temp, why do you think more oil at the same temp is going to make it run cooler? Wouldn't you want to cool the oil more rather than just have more hot oil? The fan is going to stay the same size as is the oil cooler and the pump isn't going to circulate the oil any faster. Maybe my logic is flawed here.
Why not just increase the capacity of the oil cooler or add another slim line type above the factory one? Might be a lot easier and less expensive in the long run. Otherwise, say you got another 2 liters even, how will you cool it?