From an article I wrote on Sodalitium Pianum 

[NB: The links to SP will no longer work, as the blog is defunct, but as I have the articles saved, I can repost if requested]

Of all the offenses Bishop Williamson is alleged to have made at his 2015 Mahopac, New York conference, the suggestion that one could, in certain circuмstances, still find "spiritual nourishment" at a Novus Ordo Mass was judged by his adversaries to be the worst of all. A Catechetical Refutation  defended that comment (particularly at points #7-8).

Reacting against this claim, the Pfeifferites went on to invent a new heresy, claiming against the de fidedeclarations of the Council of Trent (and the unanimous consent of the manuals) that there is no transmission of sanctifying grace to well disposed Novus Ordo communicants at a valid Mass.

To bolster that error (after the fact), they extracted two quotes of Archbishop Lefebvre from context, and held them out in a univocal sense.  Those attempts were refuted Here and Here.  Finally, an old Angelus letter of Fr. Pulvermacher was unearthed, and advanced in support of this error, which was refuted in two parts Here and Here.

Between the Catechetical Refutation and the four subsequent refutations rectifying the Pfeifferien errors on the operation of grace, we were content to have let the matter rest, having vindicated not so much the comments of Bishop Williamson, as the sacramental theology of the Catholic Church these errors attack.

However, Samuel recently posted a translation of a 1974 Econe spiritual conference of the Archbishop which leaves absolutely no room for doubt that Archbishop Lefebvre (like Bishop Williamson) believed the new Mass could still impart spiritual nourishment (i.e., sanctifying grace) to its communicants in certain circuмstances, Here.

Neither will it avail the Pfeifferites to note that the Archbishop's position on the new Mass later hardened, since what changed was not the Archbishop's theology(i.e., grace passes/grace does not pass), but his prudential decision regarding attendanceof the new Mass, given the worsening circuмstances and conditions in the Church as the fruits of Vatican II manifested themselves more clearly over the years.

[On this latter point, it is worth recalling Archbishop Lefebvre's May 9, 1980 comment in Michael Davies' classic Apologia Pro Marcel Lefebvre (Vol. II, Ch. 40) positively endorsing Novus Ordo Mass attendance that "Those who feel themselves obliged in conscience to assist at the New Mass on Sunday can fulfill their Sunday obligation" here.  I make the same observation regarding the quote the Pfeifferites pull from "Open Letter to Confused Catholics" in one of the refutations above, in which the Archbishop makes his comments on grace specific to sacrilegous and desecrated Masses, not all Novus Ordo Masses.]

As always, I suppose it is obligatory to state that that which is said above is in no sense a defense of the new Mass, but rather, a defense of Catholic sacramental theology.

Here follows Samuel's translation of Archbishop Lefebvre's conference, which leaves absolutely no room for doubt that he taught his seminarians/priests that grace can pass to Novus Ordo communicants:

"But if, on the other hand, as happens for example, they mentioned a case to me of.. some of you gave me the case of a priest who always says the old offertory, who always says the old canon, but he says the mass, he uses the new mass, he says the mass facing the people but he does not give communion in the hand. Well, if there are any seminarians that don’t have any other mass, can they attend a mass like that ? I think yes, what do you expect ! The priest who makes such an effort would be a little discouraged, hurt to see the seminarians close to him, whom he loves very much, to see that they don’t come and attend his mass under the pretext that he does not say [the old mass] absolutely from beginning to end.. I believe there are some circuмstances we have to consider !

The father of Mr Pazat who is here told me yesterday that right now, there is not a single mass of St Pius V in Madrid. If there is no more mass of St Pius V in Madrid, if one is logical with those who are strict on the question of the mass, one would have to tell all people in Madrid that they cannot put in a foot in a church, one has to be logical, one has to be logical..

Do you feel in conscience capable to tell all people in Madrid, the whole city of Madrid, all Catholics : you cannot set foot anymore in a Church ? I do not dare saying that in such an absolute manner, since there are quite a few conditions, as I will mention, quite a few circuмstances in which we cannot attend these masses.

But there are still priests who believe, there are still priests.. the mass is not always invalid, certainly not ! If it was always an invalid mass, of course we cannot go there, if it was always a sacrilegious mass, a mass regularly sacrilegious, evidently, a mass that has a net protestant tendency, it would be evident. But I think there are at the same time circuмstances in which.. we do not know, because there is still the danger on one hand of losing the faith in the case of people who don’t go to mass for one month, two months, three months, four months, a year, they will lose the faith, it’s over, that’s obvious, we cannot make ourselves any illusions, if one were to say such to a whole city, imagine !

If on the other hand obviously you say : “But they eat meat that is poisoned !” That’s true, but if one eats a meal that is more or less poisoned, they may still last a little longer, until the moment when better nourishment arrives, while if they would die of hunger, they would be dead in three weeks or a month, they would die of hunger; It would be better to die in six months than to die in one month ! It would be better if they did not die at all, of course. But what do you expect, if not going to mass causes them to die by lack of faith, if by going to a mass that is not not very good because it is poisoning them they can prolong a little.. Take someone in a cσncєnтrαтισn cαмρ who is given a choice : either you don’t eat, and thus you will die in a short time, or you will be given meat that has gone off, knowing well that you will eat bad meat, they know quite well that it will harm them, but they eat it anyway saying : “If I can survive a little longer, maybe my deliverance will come soon !” So, that is what we must say also, maybe our deliverance will come and we will have the mass of St Pius V; it is in this spirit that we have to tell them, I think.. [end of tape]"

Well, of course, if someone received devoutly the NO Communion believing it to be valid, that would undoubtedly be tantamount to a spirit communion from which many saints indicate that graces flow ... even if it’s not actually valid.

Bishop Williamson made a mistake in blurring that subjective consideration with whether or not it is offensive to God objectively speaking.

I would have answered thusly:  I am not the Church and cannot bind your conscience.  I personally hold that the NOM is offensive to God and displeases Him, to the point that it would be better not to attend it if it was all that were available ... except in extreme circuмstances.  If, however, a soul approaches the NO devoutly without having come to that conclusion, I imagine that God would provide some actual grace proportionate to their disposition, drawing them eventually back to the True Catholic Mass.  So I cannot recommend it and must advise that you stay away, but ultimately I have no authority over your conscience. 

Bishop Williamson made a mistake...

Impossible.

Agreed.

Fan Boy is as Fan Boy does.  Thank you for taking the time to clarify what was already crystal clear.

Another profound and invaluable contribution.

Agreed.

Come to think of it, +Lefebvre goes farther than +Williamson...

So, who was wrong?

I would have answered thusly:  I am not the Church and cannot bind your conscience.  I personally hold that the NOM is offensive to God and displeases Him, to the point that it would be better not to attend it if it was all that were available ... except in extreme circuмstances. 

If someone knows enough to ask a question about the NO, then they should be given the full truth of the matter (i.e. one cannot attend a doubtful mass, even in extreme circuмstances).
.

If, however, a soul approaches the NO devoutly without having come to that conclusion, I imagine that God would provide some actual grace proportionate to their disposition, drawing them eventually back to the True Catholic Mass. 

Agree, actual graces are the best chance people have, from the novus ordo. 
.

So I cannot recommend it and must advise that you stay away, but ultimately I have no authority over your conscience.

Agree with this conclusion, minus the above "exception in extreme circuмstances".  If we can't be sure that the NO is valid (and we can't), then we can't be sure it provides sacramental graces.  And we KNOW it's illicit and we KNOW it's an occasion of sin, morally and faith-wise, thus it's sinful in this regard. 

We are not discussing doubtful NOM Masses, but valid NOM Masses.

The issue here is inevitably going to be that some people here (I think you and Williamson would both fall into this category) believe that it is certain that *some* Novus Ordo masses are valid (ie. celebrated according to the book, by a valid priest) whereas I think people like Pax and some others think its questionable whether *any* novus ordo masses are valid.  While we all agree its not a good idea to attend the NOM, where one stands on the question may affect how exactly he decides to respond when put in the weeds like this.

All that said, at the end of the day, I think Ladislaus has the best answer here, mostly because I think its ridiculous for laity to take it on themselves to pretend like they have authority when they don't.  We don't have a Holy Father right now, whether you take that in the more literal, sede sense, or in the more metaphorical R and R sense of we have a spiritually *absent* Holy Father, the reality remains that we don't get to bind people's consciences on pain of sin as if we were our own popes, the most we can do is make the most prudential decisions we can for the best reasons possible and encourage others to do likewise.