My $.02 on this thread:
1. The long life coolants from Prestone and Havoline are identical and
made under license from GM, who developed the new anti corrosion
chemistry. It is ethylene glycol based, the same as the "old stuff."
EG has better heat transfer capabilities than propylene glycol, the base
used in "less toxic" coolants.
2. The carboxylate anti corrosion chemistry used does not break down
into weak acids that cause corrosion like the old additives, hence will
last much longer--at least five years/50,000 miles. They are also
phosphate and silicate free, which makes them compatible with most, if
not all, engines since they are EG based.
3. They will mix with conventional stuff, but if there is more than
10% of the old stuff left in the system the benefit of the new
chemistry is neutralized. So, flush out the old completely, or you are
wasting your money.
4. Changing the coolant less frequently is probably better for the
environment than using the slightly less toxic PG based coolants which
have to be changed every two years. All should be collected and taken
to a recycling center anyway.
5. I now use the long life in all my cars, including the '65 MGB and my
Vanagon, which has had a terrible history of head corrosion that is
usually caused by additive breakdown due to not changing the coolant
often enough. This is understandable given the pain this system is to