Dear Tom and Ray:

I bought three gallons of Prestone anti freeze several years ago but used only one. What is the shelf life of this stuff?
Will

RAY: Forever. It'll keep your shelf from freezing forever, Will.

TOM: Actually, a couple of years shouldn't make any difference to the stuff that's in the bottle. But if you put it in your radiator, I'd change it every two years or so.

RAY: Not only do you drain out any crud from your cooling system, but you also renew the rust inhibitors that tend to break down over time with use.