If it is specified as G13 then I personally would only use G13.
Get a 5-litre drum & do a full coolant swap just in case something else has been used would be my best advice, others may well disagree but generally if in doubt I would do as above.
Mixing different coolants can be a big problem, they can react with each other and turn to jelly, im not sure what prestone coolant is, i assume its a brand name, worth asking them, but my guess is dont risk.
