Well, I am going to go out on a limb here and say Antifreeze. >

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Lots of companies make the stuff, and it comes in a couple different colors: Red, and the traditional Green. Green is proabably what you want, here. >

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Prestone is a popular brand.
One thing to note is that a lot of places sell pre-mixed antifreeze these days. If you get it, then cool, just pour it in; otherwise, you are going to need to mix the stuff 50/50 with water (preferrably distilled water).
Some might recommend an additive by RedLine called
WaterWetter which I have heard good things about, though have never personally tried.
As an aside... A few things I would NOT recommend you put in your new radiator include:
Malt-O-Meal
Quick-Dry Cement
Sulfuric Acid
93 Octane Gasoline
Oreo Cookies
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This post has been edited by ConeTrouble: Jan 11, 2004 - 6:43 PM