there can be a difference with being compatible and being approved, unless an oil company pays ZF in this case to test and approve their oil, then it will not be approved, even if it is exactly the same, same manufacturers I think are claiming approval because it may be compatible. We stock Castrol oils at work, Castrol know many people were using Transmax Z with great results, but they would not recommend it themselves as they know they do not have ZF's approval