Found an old gallon jug of Peak antifreeze in my mom's garage. Its gotta be almost 20 years old, yet the foil seal is still intact over the opening. Is there any reason why I shouldn't use this in my car?

No reason I can think of not to use it.

As long as the foil seal was unbroken... I would use it.

Or keep the seal intact and store it for 10 more years and then ebay it for mad money! lol.. Just advertize it as "Classic Antifreeze for your Classic car!" Remember almost ANYTHING sells on ebay!

Or keep the seal intact and store it for 10 more years and then ebay it for mad money! lol.. Just advertize it as "Classic Antifreeze for your Classic car!" Remember almost ANYTHING sells on ebay!

Ditto that, classic old oil does too.

I would think it would be fine also. foil seal or no foil seal. I think when it comes into contact with metal is when I would think that it would start to age faster.IMHO