Whatever is on sale by the register at AutoZone or O’Reilly’s will be fine. Usually a two for one deal. It will not hurt anything.

Yes. 70k miles is . . . By far long enough.

Bad head gasket. Yes serious problem. Should be covered under warranty.

Everyday AutoZone and O’Reilly’s have the fuel system treatment stuff on sale up at the register. Whether it helps anything is debatable, but it definitely will not hurt anything and it only costs a few dollars. With that many miles run it with every tank of gas and see what happens.

Yes.