I read an article on this in Machinery Lubrication and they stated the cause was because they were cast in the 5.7l engine and were forged steel in the 6.4l engines
2004 Lincoln Town Car 111,815 miles drain/rinse/fill radiator Zerex G-05 7/8 gallon, 3/4 gal distilled water.
2002 Chevrolet Silverado 2500HD Duramax CC. 85,309 miles. Drain/rinse/fill radiator Prestone Dex-Cool 1 1/2 gal, 1 1/4 gal distilled water.