This is what John Paul II said on the topic:
As to those that follow these priests he gave this warning:
Read it all in context here:
Ecclesia Dei
The argument now is that when John Paul wrote this he did not really understand that **Lefebvre really and truly believed **that he was **compelled by the necessity **of his conscience to do what he did, and that this good faith necessity of conscience engaged the
"grave fear" exception to latae sententiae excommunication.
In this case, the “grave fear” was that John Paul was never going to give him permission to consecrate those he wanted to consecrate, so its a bit of an odd argument seeing as his “fear” was that his viewpoint would not be accepted, which is the “fear” of all dissenters. Nonetheless, that is the argument, and the link will take you to the Church documents that set forth the Church’s unaltered official stance on the matter.