A little clarification may be needed here.
The issue is mainly one of mixing the various types of coolant out there. It is here where compatibility problems pop up.
Porsche doesn't make anti-freeze, it's made by someone else. Nor is there any special proprietary voo-doo in their formula. It is basically phosphate and silicone free and the reason for this is that the phosphates attack the aluminum in the system and the silicone will mess up the oxygen sensors causing performance issues.
The chemistry is subtle though and that's where some of the incompatibilties happen, especially if you're mixing different fluids (formulations) even though they too may be phosphate/silicone free.
But, if you did a really
thorough job of flushing the system, then most any phosphate/silicone free coolant would do the job. But, that could well be the rub.
In order to do such a thorough job, you'd likely have to drain, then fill with distilled water and run the car some miles to pick-up any errant coolant still in the system (obviously this would need to be in non-freezing conditions). In fact, you may need to repeat the process in order be
certain all the old coolant was removed. Running for any length of time on pure distilled water would likely take a toll on the waterpump bearing as it uses the coolant as a lubricant between the impeller shaft and the bearing.
So while you could probably get away with it if you prepped the system as described above, it's probably better to simply go with the oem coolant - cheap insurance as someone put it.
But, many of you have used cars, and as such cannot be certain what the PO or PO's dealer may have used in the past. You may have gel forming in the system right now and not even know it. And, be aware that the coolant isn't really 'lifetime' - the 'best' coolants out there are currently 5yr./150k mi. (whichever comes first).
If I were to buy a used Boxster, one of the first things I'd do would be to flush the system and replace the fluid with oem coolant and distilled water, especially if the car were 5 yrs. or older.
One thing I'd like to see coming from Flat 6, LN Eng. or Pedro would be an in-line sleeve with a petcock that could be spliced into the lower radiator hose to make fluid changes easier for DIYers. It could be produced and sold cheaply and I suspect there's a market for it, maybe even rivalling oil filter adapters and mag plugs.
