Do not use the stuff you can buy off the shelf, there is another thread on here (which I can't find at the moment) which very clearly states that Hyundai/Kia's OEM coolant is far supirior in terms of quality and lack of 'nasty' chemical content.
I'm in the UK, so cannot comment on your warranty situation - Ours is much better!
I'm in the UK, so cannot comment on your warranty situation - Ours is much better!