I would just get the additive - it's called "Supplemental Cooling Additive" or just "SCA", and it's available at any heavy truck parts place or at dealerships like Cat/Cummins/Detroit/International, etc. There are test strips for checking that the level is correct, and it's a good idea to check a few times a year as the additives "go away" over time and need to be replenished every so often.
The "special coolants" that were referred to (like the Chevron DELO I use, or the Texaco coolant that Bjorn uses, or the Cummins coolant that Will uses) are simply coolant that already has the SCAs mixed in... which is, as far as I know, the only thing that makes those brands different from WalMart, Prestone, Peak, etc.
Regards,
Jon