[Brian in Tucson]

Green has silicates that will precipitate out when you mix it with a modern coolant.

I know it's a lot of work, but I'd flush it with hose water first to get the old coolant out. Then flush it with distilled water to dilute and remove the hose water. Then a couple of gallons of OAT coolant, I'm using Prestone dexx, and top off with distilled water. I don't think a little green in with the blue, red, or orange coolant will harm anything. But getting most of the old coolant out is a good idea.

Lots of coolants are OAT, non silicate formulas. Japanese (Toyota, Honda, etc.) & other european coolants are every bit as good as Porsche brand and can be significantly more affordable. Coolant from the VW empire all probably come from the same plant.