I was told by a tractor mechanic that the durability test was taken out of the CK-4 certification, so it would be able to pass other test. He went on to say that a test was done by getting some GM 4.3L V6's with flat tappet heads and it showed that the CK-4 oils would did not hold up and it worn the engines down. I couldn't find the anything online about it. Is it true or what? I understand that newer engines don't use flat tappet heads anymore(maybe except Ford) and that may be why it was removed.