What happens if wrong coolant was used

It's hard to say what could happen after a couple days. Sometimes the system can begin to clog up if different coolants are mixed. In your case I would diagnose the leak and repair it. Flush the system with water and a cooling system flush product you can find at all auto parts stores. Flush with water a few times to be sure its all out and then refill with the proper coolant. Alex

Unless the antifreeze was *extremely old* - it should not have caused any problems. Extremely old = content ingredients - newer antifreeze brands are pretty much universal. I don't think that caused any cooling system failures - especially in such a short time.