Found this on Lubrizols site:
What is so drastic about the GF-4 requirements that would prohibit a GF-4 oil from being used in an engine spec'd for GF-2?
quote:
The demands that ILSAC GF-4 will place on oil performance, fuel economy and engine durability are so rigorous that a new class of motor oils will have to be engineered. In fact, the upgrade of these oils will be so significant that, for the first time in lubrication history, the new oils may not be back-serviceable.
What is so drastic about the GF-4 requirements that would prohibit a GF-4 oil from being used in an engine spec'd for GF-2?