I would stick with dexcool. If you insist on using something else, then have the coolant system totally flushed with the dex before adding your other coolant.
Dexcool does not work well with other coolants. So, as Tom said, either get ALL the Dexcool out and use something else or use Dexcool. It can be difficult to get all the coolant out when doing a flush. I would replace the coolant with what is recommended which is Dexcool.
use factory recommended coolant- can't be too careful with those old caddys-
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