Pope John Paul II: "[It] is everyone's duty to work to ensure that the poor have access to credit on equitable terms and at affordable interest rates." (source) (1 January 1998)
Pope Sixtus V famously declared interest to be "detestable to God and man, damned by the sacred canons and contrary to Christian charity" (secondary source, page 7) (1586)
I know some people will immediately try to distinguish Sixtus V's comments by noting that he was referring to "usury" and then applying the modern definition of "usury" as "excessive interest."
Pope Benedict XIV anticipated this kind of attempt and wrote:
One cannot condone the sin of usury by arguing that the gain is not great or excessive, but rather moderate or small; neither can it be condoned by arguing that the borrower is rich; nor even by arguing that the money borrowed is not left idle, but is spent usefully, either to increase one's fortune, to purchase new estates, or to engage in business transactions. The law governing loans consists necessarily in the equality of what is given and returned; once the equality has been established, whoever demands more than that violates the terms of the loan. Therefore if one receives interest, he must make restitution according to the commutative bond of justice; its function in human contracts is to assure equality for each one. This law is to be observed in a holy manner. If not observed exactly, reparation must be made.(source) (1745)
Benedict XIV argued that his position was not merely his personal private opinion or a theological innovation, and prohibited teachings against his teaching:
4. This is how the Cardinals and theologians and the men most conversant with the canons, whose advice We had asked for in this most serious business, explained their opinions. Also We devoted our private study to this matter before the congregations were convened, while they were in session, and again after they had been held; for We read the opinions of these outstanding men most diligently. Because of this, We approve and confirm whatever is contained in the opinions above, since the professors of Canon Law and Theology, scriptural evidence, the decrees of previous popes, and the authority of Church councils and the Fathers all seem to enjoin it. Besides, We certainly know the authors who hold the opposite opinions and also those who either support and defend those authors or at least who seem to give them consideration. We are also aware that the theologians of regions neighboring those in which the controversy had its origin undertook the defense of the truth with wisdom and seriousness.(same source - same date)
5. Therefore We address these encyclical letters to all Italian Archbishops, Bishops, and priests to make all of you aware of these matters. Whenever Synods are held or sermons preached or instructions on sacred doctrine given, the above opinions must be adhered to strictly. Take great care that no one in your dioceses dares to write or preach the contrary; however if any one should refuse to obey, he should be subjected to the penalties imposed by the sacred canons on those who violate Apostolic mandates.
Was John Paul II the "successor" of Sixtus V and Benedict XIV? We report, you decide.
Nevertheless, I think a few people will recognize that JP2's view on interest (while apparently motivated by a concern for the poor) is not only different from, but expressly condemned by, Sixtus V and Benedict XIV. Popes make mistakes. Either JP2 was right or S5 and B14 were right, but they are not both right. Either taking interest on loans is a sin or it is not.
But some will complain that all this is trivial, because none of the staments involved are ex cathedra statements. Consequently, there is no impact on papal infallibility, even where (as here) popes seem to contradict one another. That kind of response (while it reveals a recognition of part of the problem) misses the thrust of the issue.
To get at the issue, let me ask a few further thought-questions for my papist readers. Do you believe you can decide who was right and who was wrong between JP2 and S5/B14? If so, what is your standard? Are you making yourself a mini-pope by deciding? If not - do you see why we are not making ourselves mini-popes when evaluate Trent or Vatican I or Vatican II? And our standard is the unshakable standard of Scripture, which is the only authenticable tradition of the apostles.