Solved! Go to Solution.
The article link I provided above is the only written notation I am aware of on this. You can access this if you have a PTC.com account.
Currently Creo supports up to IDX 3.0 for Creo 4.0 and Creo 8.0. The default IDX file exported from both Creo 4.0 and Creo 6.0 are of version 3.0. IDX version 3.5 is not supported
It should be documented in both of these locations IMO. You should open a ticket with PTC support and get the documentation updated with this info.