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Taken from a respected porsche authority,
"Porsche recommends using a phosphate free anti-freeze in the aluminum engines used on 944s. Standard glycol type anti-freeze which contains phosphates will cause minerals to plate out on the heat transfer surfaces inside the engine. I've read that it isn't really isn't necessary to use phosphate free anti-freeze in the United States because most water systems have a very low mineral content. However, this doesn't take into account those who have private wells. I figure better safe than sorry. Phosphate free anti-freeze used to be a little hard to find. However, it's becoming much easier to find now days. I personally prefer Prestone Extended Life because you can buy it almost anywhere and it's not that expensive relative to other anti-freeze brands. Be suspicious of anti-freeze that says, "Safe for Aluminum Engines" on the label. Unless it specifically says "Phosphate Free" on the label, it probably isn't."