[Cloudsurfer]

Is the Porsche coolant a guaranteed thing, that's guaranteed to not cause compatibility issues? Assuming that your car is currently filled with factory coolant, absolutely. As has been pointed out, the issue is not in the coolant meeting proper specs, but in being fully compatible with whatever is currently in the system.

I don't think you can even buy coolant anymore, even the cheapest stuff around, that is not phosphate and silicone free, so is Prestone Dex-Cool acceptable? Certainly more than likely. Is the cheapest Walmart brand stuff probably fine too? Yup. Am I going to try either of these in my car? Nope.

If you do a complete flush, you can virtually run whatever you want in there. The key word there is complete. It will likely take 3 drain and fill with distilled water cycles to get everything out. I recently had to do this on my newest BMW, as when I purchased the car it had "standard" green coolant of unknown origin, and it took me 3 full drain and fill cycles until the water came out clear, without hints of green. Keep in mind also, that when doing this, you'll likely have to add a bit more coolant than water when you re-fill, as there will always be residual fluid left (the reason for the fill and drain cycles in the first place), which is now just water, not 50/50 mix.

If you're just topping off the system, I would save the hassle and just get the factory stuff. If doing a full drain and re-fill, you can save a little bit of money not buying factory coolant. At the end of the day, is all that hassle worth saving $20? Not to me.