Chaps
I have read both the AD and SIN. The AD is not great from an airworthiness authoring perspective, as HC pointed out above, and it only applies to a specific group of aircraft / gear boxes.
Scotsheli I suspect you mean the SIN which states:
"As a complement to the scheduled maintenance actions, the proposal is based on an analysis of the vibration data delivered by the health systems, preferably to be performed every 4 flying hours which may be extended to 5 flying hours for operational needs."
This is not the same as "If you dont have an affected shaft, 5 hours it is. End of."
With no definition of "operational needs" then the SIN would appear to be open to interpretation, not great practice in the world of airworthiness.
I had been led to believe that the "operational needs" element has been interpreted differently between operators, if I am incorrect then I apologise - this is a place for rumour not disinfomation.