There is 2 different things here: corrosion inhibitor R1-3B and antifreeze R1-32/33.
The factory fill for the 6 (ef/el anyway) was 5% R1-3B (inhibitor)and the V8 got 50% R1-32/33 (anti-freeze).
The anti-corrosion supplement (inhibitor), at 5% for the 6, I think is for the alloy head, as the V8's have an iron head.
For the anti-freeze, Ford say good for 3 years @33% and good for 4 years @ 50%.
Put is this way, if you live where there is no frost( and your cooling system is perfect) and have a 6, you can use 5% R1-3B inhibitor.
No matter where you live, if you got a V8, you should use 50% water/antifreeze, this is not for freeze protection, it is for anti-boil protection.
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