Being a theology nerd, there are certain topics that I find fascinating (e.g., Mariology) that many other LDS do not really think much about, let alone study and write about. One area is that of Catholic Traditionalism. In my experience, Traditionalists tend to be more informed about Catholicism and more willing to discuss/debate in comparison to most of those within the “mainstream,” so I have found it useful to be informed about the SSPX, various Sedevacantist groups, the debate about the Thuc episcopal consecrations from 1981, and the debate about the Tridentine and New Order of Mass.
On the topic of the New Order of Mass (Novus Ordo Missae; AKA The Mass of Paul VI), introduced in 1969. Some Traditionalists who are not Sedevacantists (e.g., SSPX and similar groups) argue that Vatican II and the New Order of Mass was never declared to be binding. However, Paul VI soundly refuted this in 1976:
Paul VI, Address, May 24, 1976: “And the fact is all the more serious in that the opposition of which we are speaking is not only encouraged by some priests, but is lead by a prelate, Archbishop Marcel Lefebvre, who nevertheless still has our respect.
“It is so painful to take note of this: but how can we not see in such an attitude—whatever may be these people’s intentions—the placing of themselves outside obedience and communion with the Successor of Peter and therefore outside the Church? For this, unfortunately, is the logical consequence, when, that is, it is held as preferable to disobey with the pretext of preserving one’s faith intact, and of working in one’s way for the preservation of the Catholic Church, while at the same time refusing to give her effective obedience. And this is said openly. It is affirmed that the Second Vatican Council is not binding: that the faith would also be in danger because of the reforms and post-conciliar directives, that one has the duty to disobey in order to preserve certain traditions. What traditions? It is for this group, not the Pope, not the College of Bishops, not the Ecumenical Council, to decide which among the innumerable traditions must be considered as the norm of faith! As you see, Venerable Brothers, such an attitude sets itself up as a judge of that divine will which placed Peter and his lawful successors at the head of the Church to confirm the brethren in the faith, and to feed the universal flock, and which established him as the guarantor and custodian of the deposit of faith . . .
“The adoption of the new Ordo Missae is certainly not left to the free choice of priests or faithful. The instruction of 14 June 1971 has provided for, with authorization of the Ordinary, the celebration of the Mass in the old form only by aged and infirm priests, who offer the divine Sacrifice sine populo [without people]. The new Ordo was promulgated to take the place of the old, after mature deliberations, following upon the request of the Second Vatican Council. In no different way did our holy predecessor Pius V make obligatory the Missal reformed under his authority, following the Council of Trent . . .
“We have called the attention of Archbishop Lefebvre to the seriousness of his behavior, the irregularity of his principal present initiatives, the inconsistency and often falsity of the doctrinal positions on which he bases this behavior and these initiatives, and the damage that accrues to the entire Church because of them.” (L’Osservatore Romano, June 3, 1976, p. 2 as cited by Michael Dimond and Peter Dimond, The Truth about What Really Happened to the Catholic Church after Vatican II [Fillmore, N.Y.: Most Holy Family Monastery, 2007], 480-81, emphasis in original omitted)
On the issue of the Tridentine vs. New Order of Mass, for a well-researched critique from a Traditionalist Catholic, see:
Anthony Cekada, Work of Human Hands: A Theological Critique of the Mass of Paul VI (West Chester, Ohio: SGG Resources, 2010)