I personally hate Dexcool, it has tendencies to sludge and make orange gunk in the lines and has been known to cause gasket failure. If it were me, I'd have the system thoroughly flushed and green put in. That's what I did with my car.
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'13 Mustang 3.7V6 - MT82
'00 GP SE (L67 Swap, XP, Headers, 3.25")
'98 GTP, '01 Bonne