Quaeritur: I have a question.... What is the significance of Jesus' saying that he shed his blood pro multis, instead of pro omnibus? And why did we used to say pro omnibus in the Novus Ordo?
Respondeo: Here's the issue, in a nutshell. Both the traditional Missal and the new Missal officially say pro multis--that is, after all, the way Sacred Scripture has it. But the vernacular translations of the New Mass until recently had "for all" instead of the more accurate "for many." Traditional Catholics dispute the validity of the New Mass due to that mistranslation.
Fr. John McCarthy's article does a decent job of presenting and resolving the issue.