Barb,
Your system came with ethylene glycol (green automotive antifreeze). If it still has green coolant in it, and it probably does, just top it off with regular 50/50 automotive coolant. I am still running ethylene glycol (green coolant) in my 2000 Marquis, however if I ever perform a complete fluid change in the system, I will change to nontoxic boiler fluid.
If you want to change over to nontoxic boiler fluid, you will have to completely flush the system to remove all of the original coolant before installing the boiler fluid.
Gerald