Just for the record, I have , through ignorance, mixed both types of
Prestone.  The only ill effect I have noticed is that when you mix the
green stuff with the orange, you get a brownish liquid, which kind of
looks like what was in the cooling system when the van was new.

My van is a '90 Carat with maybe 160K miles on it (can't say for sure
'cause the odometer was busted for a couple of years).  I never changed
the coolant until 1994, because I was too busy figuring out why the auto
transmission kept breaking. I finally did change it (Prestone green)
when the water pump froze, although I took no particular pains to flush
out the remaining Autobahn stuff. In 1995, the first of the many hoses
blew, at which time I refilled with Prestone orange (PO4-free).

In 1996, at about 140K miles, I started getting the infamous head leak,
at which time I threw in some Alumaseal.  The leak stopped immediately.
The engine still ran.  The heater still worked. The world didn't end.  I
ran this "stew" of coolant until about a month ago, when I had to
replace the water pump again.  This time I flushed it out, refilled
with  Prestone green, and am living with a leak of maybe a tablespoon of
coolant every other day.

Observations:  I don't think it matters what you run in  the radiator,
as long as the system is bled well.  This van has gone more miles than
any other car I have ever owned (except maybe my dear departed '61
Corvair).  Sure it leaks a little from the heads, at 160K, but so do the
heads on vans that have nothing but Autobahn put in every two years.  My
opinion is that these are extremely durable and forgiving machines, so
don't worry so much.

Dan Fuller
'90 Carat
'63 Bug
'95 BMW 318IC