Which coolant do you recommend for I5 NF engine?

[George]

Also, always run distilled water.  It's cheap and assures the least
contaminates.  I have a '91 200 20 valve that I bought new almost twenty
years ago (315,000 miles now) and have had zero coolant problems, other than
the usual radiator replacements, etc. Every time I replace coolant hoses the
visible innards of the associated parts are clean and scale free........

George Sidman