Stick with stock.  Everything I've read over the years states that the
ideal coolant temp is 190 degrees.  You can cool an engine too much, and
the result is poor combustion efficiency with resulting oil
contamination.  This will not promote longevity!

This is also mentioned in a good article from another list about coolant
additives and lubricants I've posted before.  Time to post it again:

http://www.vtr.org/maintain/lubricants-redline.html#coolants

Add wetting agents if you are in a very hot climate (assuming everything
is up to par with the cooling system components.

Also, for those of you in the desert who want maximum cooling, you could
even try straight water (better heat transfer with 100% water than with
50/50 antifreeze) with wetting agents and a corrosion inhibitor like
http://no-rosion.com (which I use with my orange coolant for extra
insurance).  Just be sure it doesn't freeze!

--
Stuart MacMillan
Seattle

'84 Vanagon Westfalia w/2.1
'65 MGB (Daily driver since 1969)
'74 MGB GT (Restoring sloooowly)

Personal mechanic for:
'70 MGB GT (Daughter's)

Assisting on Restoration (and spending OPM):
'72 MGB GT (Was daughter's, now son's)
'64 MGB (Son's)

Stripped and gone but their parts live on:
'68 MGB, '73 MGB, '67 MGB GT