My take is
A) follow OEm recommendatins which means IAT
B. OAT is taking over and probably has to do with higher Aluminum content in todays engines and radiators
C) I find no conclusive evidence modern OAT is actually bad for our cars...but I have only done a half dozen different word searches to the subject...I would like clear evidence that indeed OAT is bad for pre 80s LBCs....
From:
http://www.arrowheadradiator.com/Radiator_Diagnosis,_%20Repair,_%20&_%20Replacement_%20Tips.htm
The single most important factor that leads to radiator failure is lack of cooling system maintenance. The corrosion inhibitors in conventional antifreeze are gradually depleted over time, so the recommended coolant change interval has traditionally been every two years or 24,000 to 30,000 miles for preventative maintenance. The new long-life antifreeze formulas that can go 5 years or 100,000 miles between changes reduce the need for cooling system maintenance and can reduce the risk of premature radiator failure. But most vehicles still have antifreeze with conventional additives in their cooling systems. So when regular coolant changes are neglected, corrosion goes to work.
Aluminum is more vulnerable to electrolytic corrosion than either copper/brass or cast iron because aluminum is a highly reactive metal. When the corrosion inhibitors are used up and the pH of the coolant drops to 7 or below, aluminum becomes a sacrificial anode and is eaten away.
This same type of corrosion can also occur even when the coolant is in good condition if the engine does not have a good ground connection. Voltage from the charging system will flow through the coolant to ground, creating electrolysis corrosion that attacks the components in the cooling system.
Checking the pH of the coolant with chemically-treated test strips can help you determine if the coolant is overdue for a change. The alkalinity of a typical antifreeze/water mixture will vary depending on the additives in the antifreeze and the ratio of ingredients, but is usually somewhere between 8 and 11. The average for most antifreezes is around 10.5, but when diluted 50/50 with water and added to the cooling system the pH drops to the 8.5 to 9 range. Higher is not necessarily better, though, because some of the new long-life coolants have a pH of only 8.3. Staying power is what counts.
Traditional antifreeze formulas for domestic applications with aluminum radiators, cylinder heads or blocks contain a high level of silicates (which is not really needed in cast iron engines with copper/brass radiators). Silicates form a protective barrier on aluminum that protects the metal. The additives used in European and Asian OEM antifreezes, however, are different. The Europeans use less silicates and rely mostly on borates to inhibit corrosion. Their antifreeze also contains no phosphates because hard water can react with phosphates to form calcium and magnesium sediments. The Asians use phosphates but no borates and low or no silicates because they say borates may actually add to the aluminum corrosion problem if the coolant is neglected. The new long life coolants use an entirely different corrosion inhibiting chemistry that uses carboxylate organic acids instead of the silicates, phosphates or borates. Getting rid of the silicates, some say, helps extend the life of the water pump seal because the microscopic particles of silicate in conventional antifreeze are abrasive.
Regardless of what type of antifreeze you use, it should be meet OEM specifications and be "aluminum-safe" if the radiator is aluminum.
Also see:
http://www.underhoodservice.com/Article/46495/cooling_system_antifreeze_requirements.aspx