what cooling system additives do you guys use in your radiator?

[Railroad]

None, if you need additives beyond those in normal coolant for your motor type then you need a different radiator.

Maybe, As I understand the water wetter products, it reduces the film strength of the water, like a water softener.
This allows the coolant to have better surface contact inside the engine and radiator, improving heat transfer from the surfaces to the liquid.
It may not be needed, but the advantages are there.
If you run no or low percentages of antifreeze, the wetter products contain anti corrosion agents.
What's not to like.