My personal experience only. You ''ll find all kinds of different answers. Here's mine. I wouldn't mix them. Don't like mixing stuff from one company with stuff from another. I'm not a chemist. I just don't mix. So I'd tell you to use Dex Cool to top off Dex Cool.
I heard a lot of complaints about Dex Cool. Easy to find them on the net. When I bought a '97 Lumina 6 years ago, it had Dex Cool in it. Flushed out the Dex Cool, put new hoses/thermo on it and filled it up with green anti-freeze. Prestone probably. Flushed the old green and put on new hoses and new green in a couple years ago. Tossed a new water pump on for good measure. Three of my girls used my car for a Florida trip, so I was being cautious. GM water pumps are lucky to get 100k miles, so it's good to put a new one on every 50k miles or so. They're cheap, and easy to put it. Coolant looks a bit nasty now so I'll probably do coolant again this summer.
"WalMart stuff" covers a lot of ground, but most modern formulations sold at Wallyworld or your FLAPS are labeled to be compatible with Dexcool. Check on the jug. I wouldnt be afraid to mix them under those conditions, BUT it would be best to flush out your old coolant every few years and recharge with a premium quality antifreeze (not necessarily Dexcool.. I dont like it very much, honestly) mixed 50-50 with distilled or deionized water.
It will work OK. 99% of the problems reported with Dex are due to neglect. People who never open the cooling system to check for problems or never flush the system out.
I use whatever the factory calls for. In some cases this gets expensive. Especially with foreign vehicles that use oddball coolants.
You may want to phone Prestone at 1-800-PRESTONE because their customer service people are really good at providing information. Prestone says their coolant in the yellow bottles is for "All Makes All Models", but I don't remember if they said you could mix it with Dex-cool and still get 150,000 miles & 5 years from the coolant.
Prestone does make a version of Dex-cool, and it comes in silver or grey bottles, and I bought some for $12.00 from Wal-mart on Jan. 31,
2011, the last day of a $7/gal rebate on all full-strength Prestone oolants ($5/gal on 50/50 mix, making full strength cheaper by $1/gal, assuming you use distilled water with it). Many years ago, Prestone told me that their version of Dex-cool was designed to be more compatible with solder (even mentioned Modine solder on the bottle) than GM's version was. The Texaco Havoline version of Dex-cool is supposed to be very, very similar to GM's version (Texaco supplies coolant to GM)
Here's an article from Motor magazine about all the types of antifreeze:
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